LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


Class 


TUNNEL    SHIELDS    AND    THE 

f 

USE    OF    COMPRESSED    AIR 
-••  IN    SUBAQUEOUS 

WORKS 


JAMES    HENRY   GREATHEAD. 

B.   1844.         D.    1896. 


TUNNEL  SHIELDS  AND  THE 

USE  OF  COMPRESSED   AIR 

IN  SUBAQUEOUS 

WORKS 


BY 

WILLIAM  CHARLES  COPPERTHWAITE 

M.INST.C.E.,    BRIDGES   ENGINEER  OF  THE  LONDON  COUNTY 

COUNCIL,    SOMETIME   RESIDENT   ENGINEER  OF  THE 

CENTRAL  LONDON  RAILWAY  AND  OF  THE 

GREENWICH   FOOTWAY  TUNNEL 


WITH  260  ILLUSTRATIONS   AND  DIAGRAMS 


UNIVERSITY 

OF 


NEW  YORK 

D.    VAN    NOSTRAND    COMPANY 
23    MURRAY   AND    27   WARREN    STREETS 

1906 


BUTLER  &  TANNER, 

THE  SELWOOD  PRINTING  WORKS. 

FROME,  AND  LONDON. 


PREFACE 

ALTHOUGH  the  employment  of  a  shield,  with  or  without  the  aid  of  compressed  air, 
in  tunnelling  operations  is  of  English  origin,  and  the  length  of  tunnels  so  constructed 
in  this  country  is  many  times  greater  than  the  total  amount  of  similar  work  else- 
where, the  subject  is  nowhere  dealt  with  in  Engineering  text  books  written  in 
English,  except  in  Simms'  Practical  Tunnelling  and  Prelini  and  Hills'  Tunnelling, 
which,  however,  touch  only  slightly  on  it  as  a  part  of  the  general  history  of 
tunnelling. 

Except  for  a  few  pages  in  these  works,  no  account  in  English  of  shield-work 
exists  save  in  the  form  of  papers  printed  in  the  Proceedings  of  the  Institution  of 
Civil  Engineers  and  some  description  of  current  works  which  have  from  time  to 
time  appeared  in  the  technical  journals. 

In  French  two  books  only  on  the  subject  have  appeared  :  the  very  complete 
work  by  M.  Legouez,  L'Emploi  du  Bouclier  dans  la  Construction  des  Souterrains, 
and  M.  Philippe's  Le  Bouclier,  which  gives  some  interesting  information  on  recent 
French  tunnel  works. 

The  Author  hopes,  therefore,  that  a  history  of  recent  developments  in  shield- 
work  may  be  found  of  some  use  to  his  professional  brethren,  if  only  by  collecting 
in  one  volume  a  mass  of  information  hitherto  scattered  through  many  publications, 
and  consequently  difficult  and  troublesome  of  access. 

He  has  treated  as  briefly  as  possible  the  early  records  of  the  shields,  and  of 
compressed  air  working,  holding,  indeed,  that  only  with  Mr.  Greathead  and  his 
Tower  subway  shield  the  history  of  practical  tunnelling  by  shield  really  commences, 
but  of  the  developments  witnessed  since  he  has  endeavoured  to  present  as  clear  a 
record  as  the  limits  of  one  volume  will  permit. 

Of  the  Greathead  shield  work,  the  "  assisted  shield  "  method  of  tunnelling, 
and  the  various  subaqueous  tunnels  recently  built  in  and  around  London,  he  may 
claim  to  write  from  personal  knowledge,  supplemented  by  information  generously 
placed  at  his  disposal  by  the  Engineers  engaged  in  the  various  undertakings 
described. 

Of  the  tunnels  constructed  abroad,  in  France  and  in  the  United  States,  his 

v 


1  A  Q  I 


PREFACE 

information  is  for  the  most  part  obtained  from  original  sources  or  the  writings  of 
those  who  were  themselves  actors  in  the  operations  they  describe. 

The  obligations  he  is  under  to  his  professional  colleagues  in  each  case  are 
indicated  in  treating  of  the  various  undertakings  ;  he  would  acknowledge  here, 
however,  his  special  indebtedness  to  the  Council  of  the  Institution  of  Civil  Engineers 
for  permission  to  use  the  material  contained  in  the  Minutes  of  Proceedings  of  that 
body,  and  to  the  Editors  of  the  English,  American  and  French  Engineering  journals 
by  whose  courtesy  he  has  been  able  to  reproduce  many  illustrations  of  interest. 

The  descriptions  in  this  book  of  each  undertaking  are  limited  by  considera- 
tions of  space  to  those  portions  constructed  by  shield  or  in  which  compressed  air 
was  employed,  and  only  such  further  details  as  are  necessary  for  the  understanding 
of  the  conditions  governing  the  execution  of  such  portions  are  included. 

LONDON, 

October,   1905. 


vi 


CONTENTS 


CHAPTER    I  PAGES 

THE  SHIELD  :    ITS  EARLY  HISTORY,  1818  TO  1880      .  ;  .         .          .         .  1-21 

Brunei's  Patent — The  Thames  Tunnel  Shield — The  Shield  as  Described  in  Brunei's 
Patent — Dunn's  Patent,  1849 — Guibal's  Shaft-Sinking  Machine — Rziha's 
Removable  Centres — Barlow's  Patent,  1864 — Greathead's  Tower  Subway 
Shield,  1869 — Beach's  Shield  with  Hydraulic  Rams,  1869 — Shields  at  Cin- 
cinnati and  Cleveland — Woolwich  Shield,  1874 — Woolwich  Erector — 
Antwerp  Tunnel,  1879 — Greathead,  and  the  Introduction  of  Shield  Work  in 
Recent  Years. 


CHAPTER  II 

THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK  :   ITS  EARLY  HISTORY  :   AND 

SOME  NOTES  ON  CAISSON  SICKNESS         ........         22-47 

Cochrane's  Patent,  1830 — Description  of  an  Ordinary  Airlock — Compressed  Air 
Used  at  Chalonnes,  France,  1839 — and  at  Douchy,  France,  1846 — Potts' 
Vacuum  System,  1850 — Rochester  Bridge,  1851 — Antwerp  Tunnel,  1879 — 
Hudson  River  Tunnel,  1879 — Jaminet's  Notes  on  St.  Louis  Bridge,  1868 — 
Brooklyn  Bridge,  1871 — Smith's  Proposal  for  a  Medical  Lock,  1871 — Moir's 
Lock  at  Hudson  Tunnel,  1879 — Caisson  Sickness — Conditions  of  Work  in 
Compressed  Air — Regulations  for  Controlling  Men — Effect  of  Impure  Air — 
Clauses  of  Specification  Regulating  Work  in  Compressed  Air — Experiment 
in  Purifying  Air. 


CHAPTER  III 

CAST  IRON  LINING  FOR  TUNNELS        .........        48-74 

Its  Use  in  Tunnels  Suggested  by  its  Employment  in  Pit  Shafts — Telford's  Iron 
Centres,  1824 — Rziha's  Iron  Centres  and  Face  Jacks,  1860 — Ease  of  Con- 
struction and  Immediate  Security  Ensured  by  its  Use — Circular  Tunnels 
most  Convenient  when  Cast  Iron  Lining  is  used — Proportions  of  the  Cast 
Iron  Segments — Examples  from  Recent  Work — The  Key — The  Joints — 
Central  London  Railway — Waterloo  and  City  Railway — Baker  Street  and 
Waterloo  Railway — Blackwall  Tunnel — Greenwich  Tunnel — St.  Clair 
Tunnel — Great  Northern  and  City  Railway — Rotherhithe  Tunnel — Lea 
Tunnel — Casting  of  Tunnel  Segments — The  British  Hydraulic  Company's 
Moulding  Machine — Tables — Quantities  per  Yard  Forward  of  some  Typical 
Iron  Lined  Tunnels. 

vii 


CONTENTS 

CHAPTER  IV  PAGES 

THE  GREATHEAD  SHIELD  IN  LONDON  CLAY         .......       75-134 

The  Shield — The  Assisted  Shield — General  Conditions  of  Tunnel  Work  in  London 
Clay — Movement  of  the  Exposed  Face — Type  of  Station  Shafts — Details  of 
their  Lining — Break  up  for  Shield — City  and  South  London  Shield  the  Pro- 
totype of  all  Subsequent  Machines  in  London  Clay — Detailed  Description 
of  it — The  Grouting  Pan — The  Various  Operations  of  Working  the  Shield — 
General  Observations — Speed  an  Essential — Wedges  for  Breaking  down  the 
Face — Guiding  of  the  Shield — Hay's  Patent — Cost  of  Labour  Employed — 
Glasgow  District  Subway  Shield — Central  London  Railway  Small  Shields 
— Employment  of  Pumps  on  Shield — Special  Shield  for  use  with  Thomson's 
Excavator — Thomson's  Excavator — Price's  Combined  Shield  and  Ex- 
cavator— Great  Northern  and  Strand  Shields — Arrangement  of  Shield  Rams 
and  Removal  of  Diaphragm — Greathead  Shields  of  Larger  Diameter  than 
13  Feet — Station  Shields  of  the  Waterloo  and  City  and  Central  London 
Railways — Great  Northern  and  City  Railway  and  Kingsway  Subway 
Shields — The  Segment  Erector  of  these  Latter — Method  of  Working  of  the 
Kingsway  Subway  Shield  on  a  Face  having  Ballast  at  the  Top — A  Method 
of  Supporting  a  Clay  Face  in  an  Iron  Lined  Tunnel  of  Large  Diameter. 


CHAPTER  V 

THE  SHIELD  IN  WATER-BEARING  STRATA — THE  ASSISTED  SHIELD  .          .          .      135-158 

The  Greathead  Shield  requires  Additional  Appliances  in  Water-Bearing  or  Loose 
Material — The  City  and  South  London  Railway  Shield  in  Water-bearing 
Gravel — The  Glasgow  District  Subway  Shield— The  Waterloo  and  City 
Railway  Shield — Dalrymple  Hay's  Hooded  Shield — Use  of  Clay  Pockets  on 
the  Face  in  Gravel — The  Glasgow  Harbour  Tunnels — The  Mound  Tunnels, 
Edinboro' — The  Siphons  de  Clichy  and  de  la  Concorde. 


CHAPTER  VI 

THE  SHIELD  IN  WATER-BEARING  STRATA  (continued)  ...          .          .          .      159-222 

The  Hudson  River  Tunnel — Works  in  Compressed  Air  without  Shield — A  Brick 
Tunnel  Constructed  with  an  Advance  Casing  of  Iron — Failure  of  the  Tem- 
porary Lining  of  the  Entrance — Reconstruction  of  the  Entrance  by  Means 
of  a  Caisson — The  Work  of  Tunnelling  by  means  of  a  Pilot  Heading — Sus- 
pension of  the  Works — Resumption  of  the  Works  with  a  Shield  and  Iron 
Lined  Tunnel — The  Shield  Described — The  Mechanical  Erector — Method  of 
Working — Provisions  for  Men  Suffering  from  Compressed  Air  Sickness — 
The  St.  Clair  River  Tunnel— Details  of  the  Shield— Method  of  Working— 
The  Mechanical  Erector — The  Blackwall  Tunnel — The  Caissons  Forming 
the  Shafts — Method  of  Sinking  them — Level  of  Subaqueous  Tunnel  fixed 
with  Invert  80  Feet  below  Water  Level — Details  of  Shield — The  Face 
Shutters — The  Rams — The  Hydraulic  Erectors — Method  of  Lowering  the 
Shield  from  Ground  Level  to  Bottom  of  Shaft — Compressed  Air  Machinery — 
The  Vertical  Locks  in  the  Shafts — Safety  Screen  in  Tunnel — Methods  of 
Driving  Shield — Incidents  of  the  Work — Clay  Blanket  in  River  Bed — 
Method  of  Working  the  Face  Shutters  of  the  Shield — Poling  of  the  Shield 
Invert — Conditions  of  Work  in  Compressed  Air — The  East  River  Gas 
Tunnel,  New  York — Work  without  Compressed  Air — Compressed  Air  Em- 
ployed— Compressed  Air  and  Shield  used  Together — Details  of  the  Shield. 

viii 


CONTENTS 

CHAPTER    VII  PAGES 

THE  SHIELD  IN  WATER-BEARING  STRATA  (continued)  ......      223-274 

The  Vyrnwy  Aqueduct  Tunnel — Commenced  without  a  Shield,  and  without 
Compressed  Air — Failure  of  Operations — Shield  and  Compressed  Air  Pro- 
vided— Description  of  Shield — Timber  Safety  Diaphragm  or  Trap  in  Tunnel 
behind  Shield — Second  Abandonment  of  the  Works — Reconstruction  of  the 
Shield — The  "  Trap  "  Diaphragm — Double  Airlock  used  in  the  Tunnel — 
The  Greenwich  Subway — Description  of  Tunnel — Machinery  and  Plant — 
The  Caissons — Their  Airtight  Floors — The  Plugs  in  Tunnel  Openings  of 
Caissons — Erection  and  Rivetting  of  Caissons — Sinking  of  Caissons  in  Com- 
pressed Air — Erection  of  Shield  in  Caissons — Opening  out  the  Tunnel  Face 
—The  Airlock  and  Bulkhead — Safety  Diaphragms  in  Tunnel — Rate  of 
Progress  in  Tunnelling — The  Shield,  Detailed  Description — The  "  Trap  " 
Diaphragm — The  Face  Rams — Original  Method  of  Working  a  Failure — 
Trial  of  Needles  in  the  Face — A  Poled  Face  Adopted,  with  Clay  Pockets  in 
Front  of  Cutting  Edge — Description  of  Working — Alteration  of  Shield 
Diaphragms — Ventilation  of  Shield — Cost  of  Shield,  and  Working  Gang 
Required — The  Lea  Tunnel — Shield  Chamber  and  Airlocks — Safety  Dia- 
phragms and  Vertical  Airlock — Details  of  Shield — The  Baker  Street  and 
Waterloo  Railway — Shafts  in  River — Details  of  Shield — Combination  of 
Hood  and  Shutters — Timbering  of  the  Face — Description  of  the  Method  of 
Working. 


CHAPTER  VIII 

THE  SHIELD  IN  MASONRY  TUNNELS  .          .          .          .          .          ;          .  275-  302 

The  Use  of  a  Roof  Shield  in  Masonry  Tunnels — The  Collecteur  de  Clichy  "  Extra 
Muros  " — The  Chagnaud  Shield — Detailed  Description — The  Conveyor — 
Method  of  Working  the  Shield — The  Centres  for  the  Masonry  Arch — General 
Working  Results — The  Collecteur  de  Clichy  "  Intra  Muros  " — Details  of 
the  Shield — And  of  the  Conveyor — The  Centres  for  the  Masonry — The  Lag- 
ging— Method  of  Working — General  Working  Results — The  Siphon  de  1'oise 
— The  Shield  Similar  to  the  East  River  Machine — The  Airlock — The  Con- 
crete Lining  to  the  Tunnel — Details  of  the  Iron  Centres  and  Casing — Method 
of  Driving  the  Shield  and  Constructing  the  Concrete  Lining — Concrete  Lining 
Compared  with  Cast  Iron — The  Paris  Extension  of  the  Orleans  Railway — 
Double  Line  Masonry  Tunnel — Method  of  Working  with  Advance  Headings 
for  the  Sidewalls — Details  of  the  Shield — Description  of  the  Working — The 
Centres  for  the  Masonry — General  Remarks. 


CHAPTER  IX 

THE  SHIELD  IN  MASONRY  TUNNELS  (continued)  .......      303-338 

The  Tremont  Street  Tunnel,  Boston,  U.S.A. — Work  Commenced  without  a  Shield 
— A  Roof  Shield  Decided  on — Method  of  Work — Details  of  Constructing  the 
Side  Walls— Details  of  the  Shield — The  Sliding  Shoes — Cast  Iron  Bars  Built 
in  the  Brick  Arch  to  Receive  the  Thrust  of  the  Shield  Rams — Rate  of  Pro- 
gress— The  Boston  (U.S.A.)  Harbour  Tunnel — Conditions  of  Compressed 
Air  Work — Details  of  the  Shield — Method  of  Working — Rate  of  Progress — 
The  Metropolitan  Railway  of  Paris — Comparatively  Limited  Employment 
of  Shields — Methods  of  Shield  Work  Adopted — Sections  in  which  Shields 
were  Employed — The  Champigneul  Shields — Details  of  the  Shield — Central 
Advance  Heading  LTsed — Method  of  Working — Centres  for  Masonry — Rate 

ix 


CONTENTS 

PAGES 

of  Progress — Interruption  of  Street  Traffic  Above — General  Remarks  on 
the  Champigneul  Shield — The  Lamarre  Shields — Details  of  the  Shield — 
Timber  Centres — Unsatisfactory  Results  of  Working — The  Dieudonnat 
Shields  and  the  Weber  Shields — General  Remarks  on  the  Metropolitan 
Railway  Shields. 


CHAPTER  X 

RECENT  TUNNELLING  WOKK  CARRIED  OUT  WITH  A  SHIELD  OB  WITH  COMPRESSED  AIR  339-365 
Recent  Tube  Railways  in  London — The  Rotherhithe  Tunnel,  London — General 
Description — Vertical  Locks  in  the  Shafts — The  Shield — Steel  Bulkhead 
in  Tunnel — The  River  Dee  Tunnel — General  Description — Sinking  of  the 
Shafts — Compressed  Air  Work — Driving  of  the  Tunnel — Paris  Metropolitan 
Railway  (Extension) — The  Raquet  Shield — Conditions  of  Work — Descrip- 
tion of  the  Shield — The  Brackenagh  Tunnel,  Ireland — The  Hilsea  Tunnel, 
Hampshire. 


CHAPTER  XI 

COST  OF  THE  SHIELD          .          .          .          .          .          .          .          .          .          .          .      366-374 

First  cost  of  Shield — Examples — Cost  of  Tunnelling  per  Yard  Forward,  and  per 
Cubic  Yard  of  Content — Tables  Giving  Details  of  Quantities  and  Prices — 
Comparison  of  Cost  of  Small  Tunnels  in  Masonry  or  Brickwork  and  Cast 
Iron — Increase  of  Cost  due  to  Compressed  Air.  Gangs  of  Miners — Rates 
of  Pay — Numbers  of  Men. 


APPENDIX  A 

A  CHRONOLOGICAL  LIST  OF  EVENTS  CONNECTED  WITH  TUNNELLING  BY  MEANS  OF  A 

SHIELD  OR  OF  COMPRESSED  AIR  375-382 


APPENDIX  B 

SOME  ENGLISH  PATENTS  RELATING  TO  TUNNELLING  WITH  SHIELD  AND  COMPRESSED 

AIR,   1818   TO   1904 .          .          .          .      382-384 


INDEX .  .  385-389 


LIST  OF  ILLUSTRATIONS 


NO. 

PAGE 

James  Henry  Greathead.          .          .          .          .          .  ._ 

Frontispiece 

1 

Brunei's  Thames  Tunnel  Shield  :  Longitudinal  section 

2 

2 

,,               ,,             ,,           ,,          Cross  section            .... 

...          3 

3 

Brunei's  Patent  of  1818   .         .          .      •    .          .          .          .          .          . 

5 

4 

6 

5 

Guibal's  Machine  for  Sinking  Shafts  in  Running  Sand   ". 

8 

6 

Barlow's  Shield  of  1864    .         ".          

10 

7 

Barlow's  Shield  of  1868    .         .          .          .          ...          .          . 

11 

8 

Tower  Subway  Tunnel  Lining 

12 

9 

Greathead  Shield  at  Tower  Subway           ...... 

13 

10 

Beach's  Shield          ......... 

15 

11 

Greathead  Shield  for  Woolwich  Tunnel     ...... 

17 

12 

Greathead's  Erector  for  Woolwich  Tunnel 

18 

13 

»                                            11                                                           ><                                     5»                                         ..... 

18 

14 

Antwerp  Tunnel  Lining    .      ^.          .          :          .          .          : 

19 

15 

Lea  Tunnel  Airlock      •^i          .          .          . 

24 

16 

25 

17 

Cochrane's  Patent,   1830  ......... 

27 

18 

28 

19 

Rochester  Bridge  :  Section  of  cylinders     .          .          ... 

31 

20 

„             ,,         Enlarged  section  of  cylinder           .... 

32 

21 

,,             ,,         Cross  section  of  cylinder       ..... 

33 

22 

,,             ,,         Plan  of  gearing  for  sinking  cylinder 

33 

23 

Cast  Iron  Tunnel  Lining  :  Central  London  Railway  .... 

54 

24 

,,                        ,,                              ,,                      ,,.... 

55 

25 

,,                        ,,                              ,,                      ,,.... 

56 

26 

,,                        ,,               Waterloo  and  City  Railway,  London    . 

57 

27 

,,                        „               Baker  Street  and  Waterloo  Railway,  London 

58 

28 

,,                        „               Blackwall  Tunnel,  London 

60 

29 

11                       11                             j>                 »                        ... 

61 

30 

,,                        ,,               Greenwich  Tunnel,  London 

62 

31 

11                                                     11                                                                   5>                                        11                                                     ... 

63 

32 

11                                                     11                                                                   11                                        11                                                    ... 

64 

33 

„                        „               St.  Clair  Tunnel,  Canada  .... 

65 

34 

,,                        ,,               Great  Northern  and  City  Railway,  London    . 

67 

35 

,,                        „               Rotherhithe  Tunnel,  London 

68 

36 

11                                                          11                                                                     »»                                      11                                      »                             •                         •                         • 

68 

37 

11                                                          11                                                                     11                                      11                                      »»••»• 

69 

38 

,,                        ,,                Lea  Tunnel,  London          .... 

69 

39 

11                                                          11                                                                     11                                      11                                            .... 

..       70 

40 

Moulding  Machine  for  Segments  of  Tunnel  Lining     .          . 

.        71 

41 

,,                              ,,                              ,,.... 

.        72 

42 

Central  London  Railway  :  Marble  Arch  Station          .... 

77 

43 

11             11               ,,                 ,,             ,,         ,,               .... 

78 

44 

,,             ,,               ,,           Cast-iron  shaft  lining        .   •" 

79 

45 

»             ,,               ,,                    ,,                    ,,                                      .' 

79 

46 

»             •>•>               11                   11                   11                 ...          , 

80 

47 

City  and  South  London  Railway  :  Kennington  Oval  Station 

81 

48 

Break  up  for  Shield  Chamber  in  London  Clay           .... 

83 

49 

,,                 ,.               „                   .1             ».               ..... 

84 

XI 


LIST    OF    ILLUSTRATIONS 

NO.  PAGE 

50  Break  up  for  Shield  Chamber  in  London  Clay  .  .  .  .  .        85 

51  „  „  „  „„'....          ".'         .        86 

52  City  and  South  London  Greathead  Shield :    Back  elevation         .          .          .          .        87 

53  „  ,,  ,,  ,,  ,,  Longitudinal  section         .  .89 

54  ,,  ,,  ,,  ..  ,,          Half  front  elevation         ...        90 

55  „  ,,  ,,  ,,  ,,  Details  of  hydraulic  rams         .-         .        91 

56  The  Greathead  Grouting  Pan ..95 

57  The  Greathead  Shield  in  Clay:  Method  of  working.  .          .          .  .97 

58  „  „  „  „  „ .98 

59  District  Railway,  Glasgow  ;  The  Greathead  Shield  :  Longitudinal  section     .          .      103 

60  ,,  ,,  ,,  „          ,,  ,,       Back  elevation    .          .  .      105 

61  Central  London  Railway:  Back  elevation  .          .          .  .  .          -.          v     106 

62  ,,  ,,  ,,  Greathead  Shield  :  Longitudinal  Section  and  half  front 

elevation        .          .          .          .          .          .          ...      107 

63  ,,  ,,  ,,  Greathead  Shield  for  use  with  mechanical  excavator     .      108 

64  „  „  „  „  „  „  „  „  .      109 

65  ,,  „  ,,  Thomson's  excavator         .  .          .          .          .110 

66  Charing  Cross  and  Hampstead  Railway  :  Price's  combined  shield  and  excavator     112 

67  „  „  „  „  „  „  „  „  114 

68  Great  Northern  and  Strand  Railway  :  Greathead  shield     .          .          .          .          .116 

69  „  „  „  „  „         .  ,      117 

70  Central  London  Railway  :  Station  Tunnel  Shield  ;  half  back  elevation  and  half  cross 

section  .          .          .  .         '.          .          .          .      119 

71  ,,  ,,  ,,  Station  Tunnel   Shield:   longitudinal  section       .       • -\     120 

72  ,,  ,,  ,,  ,,  ,,  ,,  half  plan  of  platform   .        ...      121 

73  Kingsway  Subway  (London)  Shield  :    Half  front  elevation  and  half  cross  section  .      123 

74  ,,  ,,  ,,  ,,  Longitudinal  section  ....      124 

75  ,,  ,,  ,,  ,,  Sectional  plan  .....      125 

76  ,,  ,,  ,,  ,,  Details  of  hydraulic  ram  .          .          .      126 

77  ,,  ,,  ,,  „  Elevation  of  segment  erector      .          .          .      127 

78  ,,  „  „  ,,  Bracket  for  segment  erector      .          .          .128 

79  ,,  ,,  ,,  „  Pivot  of  segment  erector  .          .          .      128 

80  ,,  ,,  „  ,.  Movable  arm  of  segment  erector       .          .129 

81  ,,  „  „  ,,  Details  of  segment  erector         .          .          .      130 

82  ,,  ,,  ,,  ,,  Timbering  for  ballast  face  .       -   .          .          .      131 

83  Marble  Arch  "  Shield"  :  Cross  section  of  tunnel        .          .  '    .          .         ;-.     132 
83 A       „               ,,             ,,           Longitudinal  section  of  tunnel     .          .          .          .          .133 

84  City  and  South  London  Railway  :  Greathead  shield  with  timbered  face     .          .      136 

85  „  „  ,,  ,,  Bulkhead  and  air-lock  .....      138 

86  Glasgow  District  Railway  :  Greathead    shield    with    timbered    face  :    longitudinal 

section          »          .          .          .          .          .          .  .      140 

87  ,,  ,,  ,,  Greathead  shield  with  timbered  face :  elevation  of  face     141 

88  „  „  „  ..  ,,  „  ,,  heading    .          .      141 

89  ,,  „  ,,  ,,  Bulkhead  and  air-lock    .          .          .          .^142 

90  Waterloo  and  City  Railway,  London :  Greathead    shield    with    timbered    face : 

longitudinal  section       .  .  .  .143 

91  ,,  „  ,,  ,,  Greathead     shield     with     timbered     face : 

elevation  of  face  .          .          .          .145 

92  ,,  „  ,,  ,,  Greathead  shield  with  Hay's  experimental 

hood   .          .          .          .          ...  146 

93  ,,  ,,  ,,  ,.  Hay's  hooded  shield :  longitudinal  section  147 

94  ,,  ,,  ,,  ,,  .,  .,  ..          half  cross  section    .  149 

95  ,,  ,,  „  ,,  ,,  .,  ,,  half     elevation     of 

timbered  face     .          ".          .  .      «...      150 

96  Glasgow  Harbour  Tunnels:  General  plan  and  section         .          .          .          .          .152 

97  „  „  „         Shield   .  .  .154 

98  „  „  „         Bulkhead  and  air-lock     .          .          .          ...          .      155 

99  Hudson  Tunnel,  New  York  :  New  Jersey  shaft,  and  tunnel  as  first  commenced     .      160 

xii 


LIST    OF    ILLUSTRATIONS 


NO. 

100 

101 
102 
103 
104 
105 
106 
107 
108 
109 
110 
111 
112 
113 
114 
115 
116 
117 
118 
119 
120 
L21 
122 
123 
124 
125 
126 
127 
128 
129 
130 
131 
132 
133 
134 
135 
136 
137 
138 
139 
140 
141 
142 
143 
144 
145 
146 
147 
148 
149 
150 
151 
152 
153 
154 
155 
156 


Hudson  Tunnel,  New  York :  New  Jersey  shaft,  and  method  of  construction  after 

1880 . 

,,  ,,                „               Timber  caisson  for  entrance  chamber 

,,  ,,               „              Section  of  tunnel,  and  Andersen's  Pilot  System 

,,  ,,               ,,              Shield           .          .          .          .          .          .      '    . 

,,  ,,                .,               Hydraulic  Segment  Erector  . 

St.  Clair  Tunnel,  Canada :  Longitudinal  section  .          .        v.                    . 

Shield     .  .          .          .  ._      .          .  ; 

,,  ,,             ,,                 ,,     perspective  back  elevation      . 

,,  ,,              ,,                  ,,     hydraulic  rams       .           .           . 

,,  ,,             ,,          Mechanical  erector  on  back  of  shield        .       :   . 
Blackwall  Tunnel,  London  :  Longitudinal  section     •    .  •         .-".-. 

,,  ,,              „            Section  of  shaft  with  air-locks    ..... 

,,  ,,              ,,            Shaft  with  tunnel  opening        ..... 

,,  ,,              ,,            Cutting  edge  of  shaft      ...... 

,,  ,,              ,,            Shield,  longitudinal  section       ..... 

,,  ,,             ,,                 ,,      half  cross  section           ..... 

.,  ,,              ,,                  ,,      half  back  and  front  elevations 

,,  ,,             ,,                 ,,      sectional  plan       ...... 

,,  ,,             ,,                 ,,      face  shutters         ...... 

,,  ,,             ,,                 ,,          ,,         ,,       details  of             .... 

,,  „              ,,            Hydraulic  pipe  joint        .                 9   . 

„  ,,          ~i£^       Shield,  hydraulic  rams    ....       Facing  p, 

,,  ,,              ,,                  ,,      extra  hydraulic  rams               .           .              ,, 

,,  ,,             ,,                 ,,      extension  block  for  extra  hydraulic  rams 

,,  ,,             „                 ,,      hydraulic  erector,  side  elevation    . 

,,  ,,             ..                 ,,               ,,               ,,        back  elevation    . 

,,  ,,             ,,                 ,,      details  of  hydraulic  erector     .... 

,,  „             ,,           Method  of  lowering  shield  in  shaft    .          .          .          . 

,,  ,,             ,,           Travelling  stage  behind  shield  (perspective  view) 

,,  ,,             ,,           Perspective  view  of  vertical  air-lock   .... 

,.  ,,             ,,           Sections  of  vertical  air-lock     ..... 

,,  ,,             ,,           Elevations  and  plan  of  vertical  air-lock     . 

,,  ,.              ,,            Bulkhead  and  air-lock      ...... 

,,  ,,             ,,           Safety  diaphragm   in  tunnel  (perspective  view) 

,,  .,             ,,           Damage  to  cutting  edge  of  shield     .... 

,,  ,,             ,,           Cross  section  of  tunnel  under  river      .... 

,,  ,,             ,,           Working  of  sliding  shutters  of  shield  (perspective  view) 

,,  ,,              ,,            Timberwork  in  invert  of  shield          .... 

,,  ,,             ,,           Back  perspective  view  of  shield       .... 

East  River  Tunnel,  New  York :  Longitudinal  section          ..... 

,,  ,,                  ,,              Iron  polings  of  roof        ..... 

Shield   .  .          .          .          ..         .          . 

,,  ,,                 ,,             Shield,  hydraulic  rams    ..... 

Vyrnwy  Aqueduct  Tunnel,  Liverpool :  Shield  as  first  constructed 

,    as  altered 


Greenwich  Tunnel,  London 


,,  Cast-iron  tunnel  lining 

,v          Bulkhead  with  double  air-lock    . 

Longitudinal  section      . 

Contractor's  yard  .... 

Method  of  sinking  shafts       .          . 

Method  of  commencing  tunnelling  from  shaft 

Plug  of  tunnel  opening  in  shaft 

Details  of  "  plug "         .          .          .        '  . 

Bulkhead  and    air-lock.          .. 


PAGE 

163 
164 
166 
168 
170 
171 
173 
175 
176 
178 
179 
181 
182 
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184 
188 
189 
191 
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232 
233 
234 
236 
237 
241 


Xlll 


LIST    OF    ILLUSTRATIONS 


NO. 

PAGE 

156A 

Greenwich  Tunnel,  London:  Bulkhead  and  air-locks          . 

242 

157 

,,                 ,,             ,,          Shield,  longitudinal  section     ..... 

244 

158 

,,                 ,,             ,,                „      front  elevation             .                 ~  . 

246 

159 

,,                 ,,             ,,                ,,      back  elevation             .          .          . 

247 

160 

,,                 ,,             ,,                ,,      half  sectional  plan       ..... 

248 

161 

„                  ,,              „                 ,.      method  of  working     ..... 

250 

162 

,,                 ,,             „                ,,            ,,                 ,,,.... 

251 

163 

,,                 ,,             ,,                ,,      as  altered            .          ... 

252 

164 

»                                       >5                              >»                                     »                                >J                                             ...... 

252 

165 

,,                  ,,              ,,                 ,,      hydraulic  rams  ...";.. 

254 

166 

5)                                            »                                 »                                         »5                                    55                                 »                                       ..... 

255 

167 

,,                 ,,             ,,                ,,      hydraulic  face  rams    .          .          . 

256 

168 

Lea  Tunnel,  London  :  Longitudinal  section         ....... 

258 

169 

,,          ,,             ,,          Vertical  emergency  air-lock 

259 

170 

,,          ,,             ,,          Safety  diaphragm  in  tunnel     ....      ,.   . 

260 

171 

,,          ,,             ,,          Shield,  Longitudinal  section    ...... 

260 

172 

,,          ,,             ,,               „       front  elevation     ....... 

261 

173 

,,          ,,             ,,               ,,       cross  sections       ....... 

262 

174 

,,          ,,             ,,                ,,       part  plan    .          .          .          .          . 

263 

175 

,,           „              ,,                ,,       hydraulic  face  rams      ...... 

263 

176 

Baker  Street  and    Waterloo  Railway,  London  :  Longitudinal  sections  of  tunnels 

under  Thames 

265 

177 

,,                                      ,,                          ,,          Shield,  longitudinal  section 

266 

178 

,,                                      ,,                          ,,                ,,        half  back  elevation. 

267 

179 

„                                      ,,                          ,,                ,,        arrangement  of  hydraulic 

rams  .... 

269 

180 

,,                                      ,,                          ,,                ,,        method  of  working 

270 

181 

,,                                      ,,                          ,,                „       timbered  face. 

271 

182 

,,                                      ,,                          ,,                ,,        hydraulic  rams 

273 

183 

Clichy  Main  Sewer,  Paris  :  Cross  section  .          . 

277 

184 

,,                   ,,                 ,,      Longitudinal  section         ...... 

278 

185 

„                   ,,                 ,,      Chagnaud  Shield  :  Longitudinal  section      . 

279 

186 

,,                   ,,                 ,,               „               „        cross  section         .          .          .          T 

280 

187 

„                   „                 „               „               „        shoes 

281 

188 

,,                   ,,                 ,,       Fougerolle  Shield  :  Longitudinal  section  . 

284 

189 

,,                   ,,                 ,,                   ,,             ,,                 ,,                 ,, 

285 

190 

,,                   ,,                 ,,                   ,,             ,,       cross  section        .          ... 

286 

191 

,,                   ,,                 ,,       Detail  of  wedges  of  centres      .          .          . 

287 

192 

,,                   ,,                 ,,       Detail  of  lagging  for  centres     ..... 

288 

193 

,,                   ,,                 ,,       Reduction  of  shield         .          . 

289 

194 

Syphon  under  River  Oise,  France  :  Cross  section  of  tunnel          .          .          . 

291 

195 

,,                      ,,                   ,,         Longitudinal  section    ..... 

292 

196 

,,                     „                   ,,         Method  of  making  concrete  lining   . 

293 

197 

Orleans  Railway  Extension,  Paris  :  Cross  section  of  tunnel        .... 

294 

198 

,,                                  ,,                Method  of  driving  side  headings 

295 

199 

,,                                  ,,                Shield,  longitudinal  section 

297 

200 

,,                                  ,,                     ,,        cross  sections             .... 

298 

201 

j>                                  ,,                     ,,,,,,..... 

299 

202 

Underground  Railway,  Boston,  U.S.A.  :  Plan      ,          .          .                    . 

304 

203 

,,                   ,,                        ,,              Longitudinal  section       .... 

304 

204 

,,                   ,,                        ,,              Cross  section,  Tremont  Street  Tunnel 

305 

205 

,,                   ,,                        ,,              Method  of  tunnelling  under  Tremont  Street 

306 

206 

55                                            «»                                                     55                                                         55                                              55                                      55                               55                  • 

307 

206A 

J>                                            »                                                     J>                                                         55                                               »                                     »5                               55                  • 

308 

207 

,,                   ,,                        „              Tremont      Street     shield,      longitudinal 

section     ...... 

309 

208 

„                   „                        ,,              Tremont  Street  shield,  cross  sections 

310 

xv 


LIST     OF    ILLUSTRATIONS 


NO. 

209 
210 
211 
212 
213 
214 
215 
216 
217 
218 
219 

220 
221 
222 
223 
224 
225 
226 


Underground  Railway,  Boston,  U.S.A. 


Tremont   Street  shield,    shoes 
Harbour  Tunnel,  cross  section 
Harbour  shield,  longitudinal  section 
„  cross  sections 

,,  sectional  plan 

,,  section  of  base 

,,  rollers 

Method  of  working  Harbour  tunnel 


»  ,,  ,,  Harbour  shield,  details  of  rams  and  push 

bars  ...... 

„  „  ,,  Working  arrangements  of  Harbour  tunnel 

Plan  of  Paris,  showing  tunnels  built  under  shields   ...... 

Metropolitan  Railway,  Paris  :    Champigneul  shield,  longitudinal  section     . 

»  „  „  ,,  ,,       plan     ..... 

»»  ,,  ,,  ,,  ,,       cross  section 

>»  ,,  ,,  ,,  ,,       cross   section   of   tunnel   with 

centres        .... 

»»  „  ,,  ,,  ,,       stages  of  the  work 

„  ,,  ,,       Lamarre  shield,  longitudinal  section 

„  ,,        cross  section         .... 

General  plan  and  section  . 

Proposed  finished  cross  section  .... 

Sections  of  shaft  showing  arrangement  of  air-locks 
Horizontal  section  through  air-locks  and  bonnet    . 
Air-locks  and  bonnet :  section  on  line  A  A,  Fig.  233 
,,  „  section  on  line  B  B,  Fig.  233 

The  Shield,  longitudinal  section    .... 

horizontal  section 


Rotherhithe  Tunnel,  London 


227 
228 
229 
230 
231 
232 
233 
234 
235 
236 
237 

238       „         ,;      ;;         ;; 

239 

240  „  „                         Bulkhead 
241 

242  Dee  Tunnel,  Aberdeen  :  General  plan  and  section 

„  „  Torry  shaft ;  section 

244  ,,  „  Cast-iron   lining 

245  „  „  Shield 

246  Metropolitan  Railway.  Paris  :  Raquet  shield 
247 


half  front  elevation  .... 
half  back  elevation  .... 
diagram  showing  disposition  of  framing 
sectional  plan  on  line  A  B,  Fig.  240  . 


Longitudinal  section 

Cross  section 

Detail  of  double-ended  cylinder  of 

hydraulic  ram  .... 
Details      of     bearings      of     rams, 

shown  in   position   in   Figs.    246 

and  247   

Cutting  edge          .... 
Details    of    ends    of    cylinders    of 

hydraulic  rams 
Details  of  central  joint  of  hydraulic 

ram  ..... 

Details  of  joint  of  pressure  pipe  in 

cylinder  of  hydraulic  ram 

254  Brackenagh  Tunnel,  Ireland  :  Shield  :  Longitudinal  section          .... 

255  ,,  ,,  ,,  ,,     Cross  sections   . 

256  Hilsea  Creek  Tunnel :  Longitudinal  section     - 

257  „          ,,  ,,        Details  of  timbering        ....... 

XV 


248 


249 


250 
251 

252 
253 


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360 


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Chapter   I 


THE    SHIELD:    ITS    EARLY   HISTORY,   1818  to   1880 

BRUNEL'S  PATENT — THE  THAMES  TUNNEL  SHIELD — THE  SHIELD  AS  DESCRIBED  IN  BRTJNEL'S 
PATENT — DUNN'S  PATENT,  1849 — GUIBAL'S  SHAFT-SINKING  MACHINE — RZIHA'S  RE- 
MOVABLE CENTRES — BARLOW'S  PATENT,  1864 — GREATHEAD'S  TOWER  SUBWAY  SHIELD,  1869 
— BEACH'S  SHIELD  WITH  HYDRAULIC  RAMS,  1869 — SHIELDS  AT  CINCINNATI  AND  CLEVE- 
LAND— WOOLWICH  SHIELD,  1874 — WOOLWICH  ERECTOR — ANTWERP  TUNNEL,  1879 — 
GREATHEAD,  AND  THE  INTRODUCTION  OF  SHIELD  WORK  IN  RECENT  YEARS 

THE  first  mention  in  tunnelling  operations  of  a  "  casing  or  cell  intended  to  be 
forced  forward  before  the  timbering  which  is  generally  employed  to  secure 
the  work  "  is  found  in  the  Specification  of  Patent  No.  4204  of  1818  of  Marc  Isam- 
bard  Brunei,  which  specification,  it  is  hardly  too  much  to  say,  covers  every  sub- 
sequent development  in  the  construction  and  working  of  tunnel  shields. 

The  progress  of  mechanical  science,  and  particularly  the  improvements  of 
hydraulic,  electrical  and  pneumatic  machinery  (Mr.  Colladon  is  said  however  to 
have  suggested  this  last  to  Mr.  Brunei  in  1828)  have  placed  in  the  hands  of  the 
tunnel  engineer  sources  of  power  unknown  to  the  inventor  of  1818  ;  yet  allowing 
for  these  advantages  the  most  elaborate  shield  of  to-day  is  worked  on  the  lines 
proposed  in  Brunei's  patent,  and  this  not  only  as  regards  the  shield  itself,  but 
also  in  the  use  with  it  of  cast  iron  as  a  tunnel  lining  which  for  the  first  time  is 
recommended  in  this  specification  and  is  figured  in  the  plans  attached  to  it. 

The  date  of  this  patent  of  Mr.  Brunei's  forms  therefore  the  starting  point 
at  which  the  history  of  tunnelling  by  means  of  a  shield  commences,  and  precedes 
by  twelve  years  only  the  equally  famous  patent  of  Sir  Thomas  Cochrane,  styled 
Lord  Cochrane,  which  first  described  the  application  of  compressed  air  to  shaft 
sinking  and  tunnelling  in  water-bearing  strata  ;  which  method,  used  in  conjunc- 
tion with  the  shield,  has  made  possible  the  great  tunnel  enterprises  of  the  last 
twenty  years. 

More  fortunate  than  Lord  Cochrane,  Mr.  Brunei  was  able  to  test  his  invention 
himself  and  to  bring  the  Thames  Tunnel,  the  first  constructed  by  means  of  a  shield, 
to  a  successful  conclusion  ;  but,  on  the  other  hand,  while  this  tunnel  remained 
for  thirty  years  the  only  one  so  constructed,  the  method  of  Lord  Cochrane  was, 
in  a  much  shorter  time,  in  general  use,  not  indeed  in  tunnels,  but  in  shaft  sinking 
and  in  caisson  work  in  the  foundations  of  bridge  piers  and  similar  structures. 

The  shields  used  by  Brunei  in  the  Thames  Tunnel  between  1825  and  1828 
substituted  for  the  ordinary  timber  work  of  a  tunnel  a  movable  frame  or  curtain 
capable  of  sustaining  the  working  face  and  holding  up  the  roof  of  the  excavation 
during  the  carrying  out  of  mining  operations,  and  at  the  same  time  affording  space 
within  its  shelter  for  the  construction  of  the  permanent  lining  of  the  tunnel. 


That  tunnel,  now  used  by  the  East  London  Railway,  crosses  the  Thames 
at  Rotherhithe,  and  was  originally  intended  to  serve  as  a  vehicular  tunnel,  or 


FIG.    1.     BRUNEL'S  THAMES   TUNNEL   SHIELD. 
Section  on  line  A,  A  (Fig.  2). 

rather  tunnels,  for  it  consists  of  two  brick  tunnels  side  by  side  having  a  common 
pier  in  the  centre. 

The  cross  section  of  the  masonry  was  a  rectangle,  the  actual  area  of  which 
was  greater  than  that  of  any  tunnel  constructed  since.  - 

2 


THE    SHIELD:    ITS    EARLY    HISTORY,    1818    TO    1880 

The  shield  originally  designed  under  the  erroneous  idea  that  the  materia 
to  be  met  with  in  making  the 
tunnel  was  mainly  of  a  clayey 
nature,  was  found  defective  when 
employed  in  loose  waterbearing 
material — as  might  well  be  ex- 
pected in  an  entirely  novel 
machine  —  and  although  it  con- 
tinued in  operation  during  the 
first  period  of  the  tunnel  works, 
from  1825  to  1828,  was  removed 
on  the  resumption  of  the  works 
in  1835,  and  an  improved  shield 
substituted  which  continued  in 
use  until  the  completion  of  the 
tunnel  in  1843. 

This  second  shield,  which  is 
the  one  figured  in  detail  in  Mr. 
Henry  Law's  article  on  the 
Thames  Tunnel,  published  in 
Weale's  Quarterly  of  Engineering, 
vol.  v.  1846,  was  of  the  same 
general  type  as  the  first  one,  but 
with  improvements  in  details. 

It  is  this  second  shield  which 
is  always  figured  and  described  in 
engineering  text -books  (see  Figs. 
1  and  2). 

The  dimensions  of  the  shield 
over  all  were  :  width,  37  feet  6 
inches  ;  depth,  22  feet  3  inches  ; 
and  length,  not  including  tail 
plates,  9  feet. 

It  consisted  of  twelve  frames 
of  cast  iron,  each  about  3  feet  3 
inches  wide,  capable  of  indepen- 
dent movement,  and  each  com- 
prised of  three  sections,  upper, 
middle  and  lower,  which  could 
also  be  propelled  separately. 

The  top  section  in  each  frame 
was  provided  with  roof  plates  or 
staves,  which  sustained  the  ground 
above,  and  being  chisel-shaped  in 
front,  served  the  same  purpose 
as  the  cutting  edge  (which  indeed 
was  suggested  by  them)  of  a 
modern  shield.  Each  of  the  two 


FIG.  2.     BRUNEL'S  THAMES  TUNNEL  SHIELD,  SHOWING 
THREE  OF  THE  LOWER  FRAMES  COMPOSING 

THE  SHIELD. 

Section  on  line  B,  B  (Fig.   1). 
Showing  three  of  the  twelve  frames  composing  the  shield. 


lower  sections  had  also  roof  plates  which  served  as  floor  plates  to  the  section 

3 


TUNNEL    SHIELDS 

above,  while  the  lowest  of  the  three  sections  rested  on  a  foot  or  base,  which  in 
turn  rested  on  planks,  introduced  one  at  a  time  as  the  shield  moved  forward. 
Those  planks  subsequently  formed  a  platform  on  which  the  brickwork  of  the 
tunnel  was  laid. 

Each  section  was  provided  in  front  with  fourteen  or  fifteen  poling  boards, 
held  up  by  screw  stretchers,  each  poling  having  independent  screws,  and  so  en- 
abling the  face  to  be  excavated  and  immediately  supported  in  front  of  each  poling 
separately.  By  an  ingenious  arrangement,  it  was  possible,  when  the  polings  in 
front  of  any  section  had  been  advanced  the  required  distance,  to  support  each 
poling  from  the  adjacent  sections,  and  so  allow  the  section,  the  polings  of  which 
had  so  advanced,  to  be  moved  forward,  when  the  polings  in  front  were  again 
stretched  to  it. 

By  a  somewhat  similar  disposition  of  vertical  jacks,  the  roof  plates  of  each 
section  could  be  sustained  from  the  floors  of  the  adjoining  sections  so  as  to  admit 
of  each  section  being  moved  forward,  free  from  pressure  from  above. 

The  whole  shield  was  held  up  against  the  face  by  screw  jacks  which  bore  on 
the  finished  masonry  of  the  tunnel. 

To  prevent  the  falling  in  of  the  roof  between  the  tail  of  the  shield  and  the 
finished  tunnel  when  the  shield  was  moved  forward,  iron  plates  long  enough  to 
bear  in  front  on  the  roof  plates  of  the  shield,  and  behind  on  the  top  of  the  masonry, 
were  provided. 

The  working  of  the  shield  was  based  on  the  general  idea  that,  while  it  was  im- 
possible to  excavate  over  a  face  of  such  large  area,  and  to  push  forward  the  shield 
to  support  the  new  face  sufficiently  rapidly  to  avoid  the  material  falling  in,  it  was 
possible  to  advance  piecemeal,  by  one  frame,  or  rather  by  one  section  of  a  frame 
at  a  time,  the  remainder  of  the  face  of  the  excavation  being  supported  by  the  other 
frames  (where  no  excavation  was  going  on,)  which  were  at  the  same  time  sup- 
porting the  polings  in  front,  and  themselves  sustained  by  jacks  behind  bearing 
against  the  finished  tunnel. 

In  actual  work,  it  was  the  practice  to  move  forward  at  one  time  alternate 
frames  of  the  twelve  into  which  the  shield  was  divided  ;  the  advance  made  being 
not  more  than  6  inches  at  a  time.  At  best  the  rate  of  progress  was  not  rapid,  14 
feet  being  the  greatest  length  built  in  one  week. 

Although  Mr.  Brunei,  as  the  result  of  almost  unexampled  courage  and  skill, 
managed  to  complete  the  tunnel,  and  connect  the  two  banks  of  the  river  by  a 
subaqueous  passage,  the  great  cost  of  the  work,  and  the  consequent  disappointing 
financial  results,  undoubtedly  checked  for  a  long  period  any  further  enterprises  in 
similar  situations. 

For  twenty-six  years  he  found  no  imitator,  but  when  in  1869  the  Tower  Subway 
was  built  by  Mr.  Greathead,  who  must  be  regarded  as  the  pioneer  in  all  modern 
work  of  this  class,  the  shield  used  embodied  the  main  features  of  the  earlier  machine. 

Indeed  the  shield  of  1869  bore  in  its  general  appearance  more  resemblance  to 
the  first  of  the  two  shields  figured  in  the  Brunei  patent  of  1818  than  to  the  actual 
shield,  or  rather  shields,  used  in  the  Thames  Tunnel. 

If  in  Figs.  3  and  4,  which  are  reproduced  from  the  drawing  attached  to 
Mr.  Brunei's  patent,  we  imagine  the  separate  cells  to  be  bolted  together  instead 
of  sliding  freely,  the  one  on  the  other,  and  the  outside  plates  to  be  joined  up  and 
so  made  into  one  cylindrical  skin,  we  should  have  a  machine  very  much  resembling 
the  Barlow,  or  the  Beach,  or  the  later  shields  figured  in  Chapter  IV. 

4 


THE    SHIELD:    ITS    EARLY    HISTORY,    1818    TO    1880 


The  manner  in  which  the  shield  of 
1818  was  intended  to  work  may  be 
given  in  Mr.  Brunei's  own  words  (ex- 
tract from  patent  Specification)  : — 

I  shall  premise  by  observing  that  the 
chief  difficulties  to  be  overcome  in  the  exe- 
cution of  tunnels  under  the  beds  of  great 
rivers  lie  in  the  insufficiency  of  the  means  of 
forming  the  excavation.  The  great  deside- 
ratum,therefore,  consists  in  finding  efficacious 
means  of  opening  the  ground  in  such  a  man- 
ner that  no  more  earth  shall  be  displaced 
than  is  to  be  filled  by  the  shell  or  body  of 
the  tunnel,  and  that  the  work  shall  be 
effected  with  certainty. 

The  first  method  I  shall  describe  for  ob- 
taining this  desirable  result  is  applicable  to  a 
tunnel  of  large  dimensions  as  well  as  to  a 
simple  drift  or  a  driftway.  In  the  formation 
of  a  drift  under  the  bed  of  a  river,  too  much 
attention  cannot  be  paid  to  the  mode  of 
securing  the  excavation  against  the  breaking 
down  of  the  earth.  It  is  on  that  account 
that  I  propose  to  resort  to  the  use  of  a  casing 
or  a  cell,  intended  to  be  forced  forward  before 
the  timbering  which  is  generally  applied  to 
secure  the  work.  This  cell  may  be  similar 
to  one  of  those  represented  in  Fig.  1,  see 
letter  C.  The  workman  thus  inclosed  and 
sheltered  may  work  with  ease  and  in  perfect 
security.  It  is  obvious  that  the  smaller  the 
opening  of  a  drift,  the  easier  and  the  more 
secure  the  operation  of  making  the  excava- 
tion must  be.  A  drift  on  dimensions  not  ex- 
ceeding 3  feet  in  breadth  by  6  feet  in  height, 
forms  an  opening  of  18  feet  area  ;  whereas 
the  body  of  a  tunnel  on  dimensions  suffi- 
ciently capacious  to  admit  of  a  free  passage 
for  two  carriages  abreast  cannot  be  less  than 
22  feet  diameter,  consequently  about  twenty 
times  as  large  as  the  opening  of  a  small  drift. 
One  of  the  modes  which  I  propose  to  follow 
for  the  purpose  of  forming  excavations  suit- 
able to  tunnels  of  large  dimensions  consists 
in  rendering  the  operation  nearly  similar  to 
that  of  forming  a  small  drift,  as  being  the 
most  easy  and  the  most  secure  way  of  pro- 
ceeding. The  apparatus  represented  in  the 
Fig.  1,  2,  3  and  4,  is  one  of  the  nature  above 
described,  and  whereby  a  large  excavation 
suitable  to  the  dimensions  of  the  proposed 
tunnel  may  be  made.  This  apparatus  is  in- 
tended to  precede  the  body  or  shell  of  the 
tunnel,  and  it  is  represented  as  if  the  work 
had  already  been  commenced  with  a  part  of 
the  tunnel  a,  a,  a,  a,  Figs.  2  and  4,  formed 
behind  it.  Fig.  1  represents  a  transversal 
view  of  an  apparatus  composed  of  small  cells 
A,  B,  C,  D,  E,  F,  G,  H,  J,  K,  lying  alongside 
of  and  parallel  with  each  other.  Each  cell 
may  be  forced  forward  independently  of  the 
contiguous  one,  so  that  each  workman  is 
supposed  to  operate  in  a  small  drift  indepen- 


TUNNEL    SHIELDS 


previously  been  moved, 
in  its  new  situation. 


dently  of  the  adjacent  one.  The  front  of  the 
work  is  protected  by  small  boards  /,  /,  /,  /, 
which  the  workman  applies  as  he  finds  most 
convenient.  Several  men  may  work  at  the  same 
time  with  perfect  security,  and  without  being 
liable  to  any  obstruction  from  each  other. 

Fig.  2  represents  a  longitudinal  section  of 
the  apparatus,  wherein  a  workman  is  seen  at 
work.  Each  cell  is  to  be  moved  or  forced  for- 
ward by  any  mechanical  aid  suitable  to  the  pur- 
pose, but  I  give  the  preference  to  hydraulic 
presses  M,  M,  Fig.  2,  which  are  made  to  abut 
against  a  strong  framing  N,  N,  N,  fixed  within 
the  body  or  shell  of  the  tunnel.  When  the  ground 
has  been  removed  and  the  several  cells  have 
been  forced  forward  to  a  sufficient  distance,  then 
a  space  P,  P,  corresponding  with  that  distance, 
is  left  between  the  ends  of  the  cell  and  the  shell 
of  the  tunnel  a,  a,  a,  a.  But  in  order  to  prevent 
the  breaking  down  of  the  earth,  or  the  eruption 
of  a  large  body  of  water  to  each  of  the  cells,  and 
on  that  side  of  each  which  is  exposed  to  the 
pressure  of  the  earth,  I  apply  strong  iron  plates 
extending  beyond  the  cell,  so  as  to  overlap  the 
shell  of  the  tunnel  previously  made,  thus  protect- 
ing the  space  already  cleared.  The  body  or  shell 
of  the  tunnel  may  be  made  of  brick  or  masonry, 
but  I  prefer  to  make  it  of  cast  iron,  which  I 
propose  to  line  afterwards  with  brickwork  or 
masonry. 

Fig.  3  represents  a  transverse  sectional  view 
of  the  tunnel  showing  the  framing  which  is  to 
form  the  abatement  for  the  hydraulic  presses. 

Fig.  4  is  a  plan  exhibiting  the  internal  ar- 
rangements of  the  cells,  the  hydraulic  presses 
M,  M,  M,  M,  and  the  framework  N,  N,  N, 
forming  the  abatement  of  those  presses.  Each 
cell  in  the  longitudinal  direction  thereof  is  formed 
into  a  prismatic  figure  such  as  adapts  itself  to 
the  situation  which  it  respectively  occupies  in 
the  area  which  is  intended  to  be  excavated.  In 
order  to  facilitate  the  progressive  movement  of 
the  cells,  I  introduce  friction  rollers  between  the 
opposite  sides  of  all  the  cells,  one  row  of  which, 
for  the  sake  of  exemplification,  is  represented  at 
P,  Fig.  2.  As  the  construction  of  the  hydraulic 
presses  is  well  understood  by  mechanics  in 
general,  and  as  the  application  and  use  of  the 
said  presses  and  framework  forming  the  abate- 
ment to  the  same  must  be  sufficiently  apparent 
from  the  preceding  description  thereof  and  the 
various  figures  in  the  annexed  drawings,  they 
require  no  further  explanation  ;  I  have  only  to 
add,  that  after  so  much  of  the  earth,  as  before 
described,  has  been  removed  by  the  workman, 
and  after  the  cells  have  been  forced  forward  by 
the  aid  of  the  presses  into  the  position  as  repre- 
sented in  Figs.  2  and  4,  and  also  after  another 
portion  of  the  shells  of  the  tunnel  has  been  added 
so  as  to  occupy  or  fill  up  the  space  p,  p,  it  then 
becomes  necessary  that  the  framework  or  abate- 
ment N,  N,  N,  should  be  moved  forward  through 
a  space  equal  to  that  through  which  the  cells  had 
The  said  frame  having  been  so  moved,  it  must  again  be  firmly  fixed 


THE    SHIELD  :    ITS    EARLY    HISTORY,    1818    TO    1880 

The  above  extract  describes  the  essential  features  of  all  the  shields  which  have 
been  constructed  since  1869,  and  perhaps  the  comprehensive  character  of  Mr. 
Brunei's  invention  can  best  be  described  by  saying  that  while  it  covered  in  its  main 
features  the  comparatively  simple  machine  designed  by  Mr.  Greathead  for  tunnelling 
in  the  London  Clay,  which  in  its  own  sphere  also  remains  to-day  the  best  and  in- 
deed the  only  apparatus  in  use  for  that  work  ;  the  more  complicated  apparatus 
used  in  the  St.  Clair  and  Blackwall  tunnels  also  derive  their  essential  features  from 
the  same  original. 

It  is  of  course  easy  to  criticise  the  details  both  of  the  shield  as  patented  by 
Brunei  and  of  that  actually  used.  Their  complication  of  parts,  and  the  division 
and  subdivision  of  the  working  face  are  defects,  but  on  the  other  hand,  as  M.  Legouez 
has  well  pointed  out,  many  of  those  features  of  the  shield  are  due  to  the  insuffi- 
ciency of  the  mechanical  appliances  at  Brunei's  disposal,  and  one  may  well  add 
that  many  of  Brunei's  details  have  reappeared  in  the  more  powerful  shields  of  later 
times.  The  same  writer  points  out,  for  example,  that  the  skin  of  the  shield,  the 
rams  which  advance  it,  the  polings  dividing  up  the  working  face  into  small  sections, 
and  even  the  use  of  clay  in  front  of  the  cutting  edge,  are  all  in  existence,  in  a 
rudimentary  form  sometimes  it  is  true,  in  Brunei's  shield.1 

The  author  may  remark  here  that  in  1900,  when  engaged  in  preparing  for 
Sir  A.  Binnie  the  plans  for  a  tunnel  about  11  feet  in  diameter  under  the  River  Lea 
in  connexion  with  the  main  drainage  system  of  London,  he  had  brought  under 
his  notice  a  model  of  a  shield  practically  similar  in  design  to  the  one  in  Figs.  1  and 
2,  except  that  the  outside  skin  was  comprised  of  "  needles  "  or  metal  strips  about 
9  inches  wide.2  This  was  invented  by  a  miner  in  the  employ  of  the  London  County 
Council,  and  was  actually  tried  a  year  later  in  the  construction  of  a  sewer  about 
7  feet  in  external  diameter  in  the  Isle  of  Dogs.  The  results  were  unsatisfactory, 
but  perhaps  had  the  Thames  Tunnel  been  circular  in  section,  instead  of  rect- 
angular, the  system  of  working  in  independent  compartments  might  have  equally 
failed  there.  On  this  point  Mr.  Greathead  (Proc.  Inst.  C.E.,  vol.  cxxiii.  p.  55) 
says  :  "  In  the  Thames  Tunnel,  Brunei  adopted  a  rectangular  section,  probably 
as  being  more  suitable  for  his  form  of  shield."  Mr.  Law,  in  the  paper  referred  to 
above,  says  that  the  rectangular  form  was  adopted  on  account  of  the  better 
resistance  it  offered  to  constantly  varying  pressures  due  to  the  rise  and  fall  of 
the  tides. 

The  second  type  of  shield  described  and  figured  in  the  specification  of  1818 
is  the  one  suggested,  as  described  by  Brunei  himself,  by  the  screwlike  action  of 
the  Teredo  Navalis,  a  marine  worm  which  can  pierce  the  hardest  woods.  The 
shield  was  never  practically  tested,  and  it  is  one  of  fame's  little  ironies  that  Brunei 
is  popularly  supposed  to  have  derived  his  great  invention  from  his  observation 
of  a  natural  excavating  machine,  whereas  in  fact  the  actual  shield  used  by  him 
borrowed  nothing  from  any  previously  known  natural  or  other  mechanism. 

The  next  tunnel  constructed  by  means  of  a  shield  was  the  Tower  Subway  under 
the  Thames  constructed  in  1869,  but  prior  to  this  date  several  inventions  were 
put  forward  on  similar  lines  to  Brunei's. 

In  1849  a  Mr.  Samuel  Dunn,  of  Doncaster,  took  out  a  patent  (No.  12,634 
of  1849)  for  a  tunnelling  machine,  for  working  in  soft  sand  and  mud,  which,  how- 
ever, was  never  tested  by  actual  work. 

1  Legouez,  Emploi  du  BoucUer,  p.  43.  2  Patent  7374  of  1890. 

7 


TUNNEL    SHIELDS 


It  consisted  of  a  cylindrical  or  elliptical  shield,  having  its  front,  which  was 
entirely  closed,  formed  somewhat  like  a  ploughshare. 

The  rear  portion  contained  in  the  cylindrical  skin,  which  was  sufficiently  long 
to  overlap  the  tunnel  already  constructed,  formed  an  hydraulic  or  atmospheric 
ram,  the  piston  of  which  had  a  head  the  full  size  of  the  shield,  and  when  a  bearing 
was  taken  on  the  tunnel  already  constructed,  forced  the  whole  shield  forward. 

The  making  of  a  piston  of  such  dimensions  properly  watertight  would  offer 
some  difficulties,  and  perhaps  with  some  idea  of  this,  the  inventor  suggested  as  an 
alternative  construction  that  a  smaller  hydraulic  ram  should  be  fixed  in  the  centre 
of  the  plough,  in  the  axis  of  the  tunnel,  and  that  its  piston  should  bear  with  radial 
arms  in  the  tunnel  lining  already  built. 

The  general  design,  as  shown  in  the  drawings  accompanying  the  specification 
of  the  patent,  is  very  crude,  and  the  main  feature,  the  plough  front,  quite  unwork- 
able ;  but  the  inventor  puts  forward  for   the   first  time  the  suggestion  that  the 
machine  should  move  in  one  piece,  which  is  a  characteristic  of  all  modern  shields. 
In  1857,  a   Mons.    Guibal   devised   for  sinking   shafts  in  running   sand,  and 
similar  material,  a  shield,  which,  except  that  its  movement 
was  vertical,  instead  of  horizontal,  was  in  all  respects  an 
adaptation  of  Mr.  Brunei's  machine.1 

In  Fig.  5  a  shaft  is  shown  partly  constructed,  in  the 
centre  of  which  is  hung  a  pipe  A.  At  the  lower  end  of 
this  pipe  are  fitted  to  it  segmental  frames  or  chambers 
B,  B,  capable  of  separate  movements.  The  chambers 
together  cover  the  area  of  the  bottom  of  the  shaft,  the 
plates  forming  their  outside  skins  are  extended  upwards 
so  as  to  surround  the  lower  end  of  the  shaft  already  built, 
and  are  provided  with  leather  flaps  C,  C,  making  a  more 
or  less  watertight  joint.  The  chambers  are  supported  from 
the  shaft  above  by  means  of  hydraulic  jacks  D,  D. 

By  means  of  a  spoon  bore  the  material  below  the 
chambers  is  removed  through  the  tube  A,  and  the  cham- 
bers are  successively  forced  down  by  the  jacks  D,  D. 

When  the  chambers  are  all  sunk  a  convenient  distance,  the  lining  of  the  shaft 
is  brought  down,  and  that  done,  the  pipe  in  the  centre  is  again  lowered,  and  the 
process  of  excavation  recommenced. 

This  system  of  shaft  sinking  has  not  been  extensively  used,  as  it  is  only 
applicable  in  very  wet  sand,  and  even  then  the  rate  of  progress  is  not  rapid.2 

In  the  early  sixties,  a  M.  Rziha  introduced  a  system  of  tunnelling  with  iron 
removable  centres  which  had  some  success,  but  which  since  the  reintroduction 
of  the  shield  system  has  been  little  used.3 

In  1864  Mr.  P.  W.  Barlow  took  out  a  patent  (No.  2207)  for  an  improved  method 
of  constructing  and  working  railways  and  in  constructing  railway  tunnels. 
In  the  specification  he  describes  his  invention  as  follows  : — 

In  constructing  tunnels  for  railways,  particularly  where  the  tunnels  are  to  pass 
under  rivers  or  under  towns  and  places  where  the  upper  surface  cannot  without  serious 


FIG.  5.  GUIBAL'S  MACHINE 
FOB  SINKING  SHAFTS  IN 
RUNNING  SAND. 


1  Drinker's  Tunnelling,  p.  742. 

2  In  sinking  an   18  foot  shaft  through  wet  sand 


r    the  Kennington  Road  for  the 

Baker  Street  and  Waterloo  Railway,  London,  Mr.  Dalrymple  Hay  has  recently  (May  1905) 
employed  a  rudimentary  shield  with  very  satisfactory  results. 
3  Drinker's  Tunnelling,  p.   677. 

8 


THE    SHIELD:    ITS    EARLY    HISTORY,    1818    TO    1880 

injury  be  broken  up  or  interfered  with,  a  cylinder  of  somewhat  larger  internal  diameter 
than  the  external  diameter  of  the  intended  tunnel  is  employed,  such  cylinder  being  by 
preference  of  wrought  iron  or  steel.  The  forward  edge  of  this  cylinder  is  made  compara- 
tively thin.  Within  this  cylinder  and  near  the  forward  end  thereof,  are  upright  plates  parallel 
to  each  other,  also  formed  with  cutting  forward  edges  in  order  to  cut  freely  through  the  soil  in 
front  when  the  cylinder  is  forced  forward.  The  earth  is  continuously  removed  from  within 
this  cylinder,  and  the  cylinder  is  from  time  to  time  forced  forward  a  short  distance  to  admit  of 
a  ring  of  iron  being  put  together  within  the  inner  end  of  the  cylinder,  such  iron  rings  being  of 
a  strength  suitable  for  forming  a  permanent  lining  to  the  tunnel.  It  is  desirable  that  the 
thickness  of  the  iron  of  the  cylinder  should  be  as  little  as  may  be,  in  order  that  the  space 
between  the  outer  surfaces  of  the  rings  and  the  earth  which  surrounds  them  may  not 
produce  any  subsidence  in  the  surface  of  the  land  above. 

****** 

The  cylinder  is  from  time  to  time  forced  forward  by  screws,  and  the  rings  of  the  iron  tunnel 
are  then  put  together,  whilst  the  surrounding  earth  is  upheld  by  the  cylinder.  If  the  soil  is 
weak,  provision  may  be  made  for  using  poling  boards  as  is  well  understood.  The  space,  as  it 
is  left  between  the  earth  and  the  exterior  of  the  tunnel,  may  be  filled  by  injecting  or  running 
in  fluid  cement. 

Fig.  6  is  prepared  from  the  drawing  filed  with  the  specification,  and  shows  a 
shield  moving  forward  in  one  piece,  but  in  all  other  respects  resembling  the  Brunei 
Patent  of  1818. 

The  suggestion  that  the  space  left  void  behind  the  cylindrical  skin  of  the  shield 
when  the  latter  is  moved  forward  should  be  filled  with  cement  is  put  forward  for 
the  first  time. 

As  is  well  known  the  apparatus  devised  by  Mr.  Greathead  for  injecting  grout" 
behind  the  tunnel  lining  is  everywhere  used  in  this  class  of  work,  and  perhaps  has 
done  more  than  any  other  invention  except  the  shield  itself,  to  render  tunnelling 
under  shield  in  the  London  Clay  practicable.1 

In  the  Tower  Subway  the  filling  with  grout  was  done  by  means  of  a  hand 
syringe  and  was  not  satisfactory  ;  the  grout  having  to  be  put  in  in  too  fluid  a  con- 
dition for  setting  well,  and  the  syringe  not  having  enough  pressure  to  force  the 
grout  into  the  smaller  interstices  in  the  clay.2 

In  1866  a  Mr.  R.  Morton,  of  Stockton-on-Tees,  took  out  provisional  protection 
for  a  shield  described  as  under 3  : — 

A  tubular  shield  of  cast  or  wrought  iron,  or  of  any  suitable  metal,  circular  or  elliptical 
in  form,  and  somewhat  larger  than  the  proposed  tunnel.  The  front  of  this  shield  is  sharp 
or  pointed  like  a  wedge.  Inside  of  the  shield  I  fix  hydraulic  presses  (one  or  more)  ;  the  pumps 
of  these  presses  are  worked  by  steam  power  in  the  usual  way.  I  fix  a  strong  table  of  cast  iron 
or  any  suitable  metal  on  the  outer  ends  of  the  hydraulic  rams  to  take  the  thrust  or  pressure. 
I  form  segmental  rings  of  cast  iron  in  suitable  widths,  with  which  I  build  the  tunnel  as  excavated 
by  the  shield,  these  rings  having  internal  flanges  with  which  they  are  bolted  together.  They 
are  made  small  enough  to  pass  within  the  shield  and  leave  a  proper  space  all  round  for  packing. 
The  shield  will  always  overlap  the  rings  2  feet,  and  in  the  space  between  I  place  india-rubber 
tubes,  expanded  by  pumping  air  or  water  into  them,  thus  filling  the  space  and  keeping  out 
water  or  mud  from  the  tunnel  ;  or  I  use  an  india-rubber  ring  or  cupped  leather  fixed  to  the 
after  part  of  the  shield  outside.  On  the  bottom  of  the  segmental  rings  I  fix  longitudinal  girders 
to  suitable  flanges  for  carrying  the  roadway. 

This  provisional  patent  appears  to  have  remained  entirely  unknown,  for  in 
the  discussions  which  subsequently  arose  as  to  the  paternity  of  the  modern  shield 
no  reference  has  ever  been  made  to  this,  which  at  any  rate  appears  to  anticipate 
Mr.  Beach's  use  of  hydraulic  power  in  his  New  York  shield. 

In  1868  Mr.  Barlow  provisionally  protected  another  design  of  a  shield,  the 

1  Patent  No.  5221  of  1886.  2  Proc.  Inst.  C.E.,  Vol.  cxxiii.  p.   62. 

3  Patent  No.  770  of  1866. 


10 


THE    SHIELD  :    ITS    EARLY    HISTORY,    1818    TO    1880 

main  feature  of  which  was  the  provision  of  a  transverse  partition  or  diaphragm 
having  in  its  centre  an  opening  capable  of  being  closed  at  will.  The  object  of  this 
diaphragm  which  closed  the  tunnel  above  the  level  of  the  top  of  the  door  was  to 
insure  that  in  the  event  of  an  inrush  of  water,  the  air  in  the  upper  portion  of  the 
tunnel  would  be  unable  to  escape,  and  an  air  chamber  be  formed  in  which  the  miners 
could  remain  until  rescued  (see  Fig.  7). 


LONGITUDINAL    ELEVATION.  CROSS 

FIG.  7.     BARLOW'S  SHIELD. 
From  Provisional  Patent  taken  out  in   1868. 


ING    TUNNEL 
SECTION     A. 8. 


The  main  features  of  the  two  patents — namely  (1)  the  cast-iron  lining  to  the 
tunnel  and  grouting  outside  of  it,  (2)  the  cylindrical  overlapping  skin  of  the  shield, 
(3)  the  use  of  screws  or  presses  to  move  the  shield  forward,  and  (4)  the  transverse 
diaphragm  of  the  1868  invention,  or  the  open  rectangular  frame  of  the  earlier — 
have  been  reproduced  in  all  shields  used  since,  and  in  a  sense  Mr.  Barlow's  design 
must  be  considered  as  the  type  from  which  the  Greathead  and  Beach  shield  are 
modelled.  It  is  true  that  it  was  itself  derived  from  the  earlier  invention  of  Brunei, 
but  some  of  its  arrangements,  notably  the  transverse  diaphragm,  the  movement  in 
one  piece,  and  the  grout  filling,  are  modifications  of  such  importance  as  almost  to 
constitute  a  new  invention. 


The  Tower  Subway 

Mr.  Barlow's  designs,  in  the  forms  set  forth  in  his  patents,  were  never  put  into 
practical  shape  by  him,  and  even  the  Tower  Subway  under  the  Thames,  of  which 
he  was  the  original  promoter,  was  built  with  a  shield,  of  similar  character  indeed 
to  his  1868  patent,  but  designed  by  Mr.  Greathead. 

This  Subway  is  interesting  historically,  as  being  the  first  tunnel  built  of  cast 
iron,  with  grout  filling  behind,  and  also  as  the  first  tunnel  of  any  kind  constructed 
with  a  shield  movable  in  one  piece.  It  is,  in  fact,  the  model  on  which  all  similar 
work  carried  out  since  has  been  designed. 

Its  successful  construction  was  due  entirely  to  Mr.  Greathead,  who  took  the 
contract  for  the  entire  scheme  from  the  Company  which  had  obtained  powers  from 
Parliament  to  construct  it,  devised  himself  the  plant  and  equipment  for  the  work, 
and  personally  superintended  its  execution. 

ii 


TUNNEL    SHIELDS 

It  is  constructed  in  its  entire  length  in  the  London  Clay,  with  a  minimum 
cover  under  the  river  of  22  feet,  and  no  difficulty  was  met  with  in  carrying  out  the 
work,  either  in  the  tunnel  itself  or  in  the  shafts,  from  water,  nor  from  loose  material. 

The  cast  iron  tunnel  lining  is  shown  in  Fig.  8.  Its  external  diameter  is  7  feet 
If  inches,  and  the  thickness  of  the  casting  over  all  is  3  inches  except  at  the  horizontal 
flanges,  which  are  4  inches  deep. 

Each  ring  is  18  inches  wide,  and  consists  of  three  equal  segments  and  a  key 
piece. 

The  weight  per  yard  forward  is  about  1  ton  6  cwt. 

The  shield  (see  Fig.  9)  l  consisted  of  a  cylinder  of  iron  plates  |  inch  thick,  and 
about  4  feet  9  inches  long.  It  was  made  with  a  slight  taper  ;  that  is,  it  was  not  a 


FIG.  8.     TOWER  SUBWAY  UNDER  THE  THAMES. 
Cast  Iron  Lining. 

true  cylinder,  the  diameter  in  front  being  slightly  larger  than  at  the  back,  with  an 
idea  of  reducing  the  skin  friction  of  the  surrounding  clay  when  the  shield  was  in 
movement. 

The  front  of  this  cylinder  was  stiffened  by  a  cast-iron  ring,  bolted  to  it  and 
made  with  a  rounded  edge  forward,  instead  of  the  now  usual  acute,  or  cutting 
edge. 

Behind  this  cast-iron  ring  was  fixed  a  bulkhead,  or  diaphragm,  of  f  inch  plates, 
having  in  it  a  doorway  or  opening,  reaching  nearly  to  the  top  of  the  shield,  through 
which  the  miners  could  pass  to  the  face.  This  doorway  could  be  closed  if  necessary 
by  dropping  across  it  3-inch  planks,  the  ends  of  which  could  be  held  by  the  vertical 
channel  irons  forming  the  jambs. 

1  No  detailed  drawing  of  this,  the  earliest  of  the  modern  shields,  exists.  The  figure  is 
obtained  from  a  sketch  prepared  in  Mr.  Greathead's  office  in  1895. 

12 


TUNNEL    SHIELDS 

Behind  the  diaphragm  again  were  segmental  castings  forming  a  cast-iron 
internal  stiffening  ring,  which  gave  solidity  to  the  skin  or  envelope  of  the  shield, 
and  at  the  same  time  carried  the  screw  jacks  which  propelled  the  shield. 

These  screw  jacks  were  worked  by  ratchet  braces,  and  bore  on  the  last  ring  of 
the  cast  iron  lining  already  built. 

With  this  machine  a  maximum  speed  of  9  feet  per  day  of  twenty-four  hours 
was  attained. 

The  substitution  of  an  opening  reaching  nearly  to  the  top  of  the  shield  in  place 
of  the  central  opening  proposed  by  Barlow  in  his  patent  of  1868  can  hardly  be 
regarded  as  an  improvement.  No  doubt  access  to  the  face  was  easier,  but  on  the 
other  hand,  the  raising  of  the  top  of  the  doorway  did  away  with  the  safety  diaphragm 
above.  With  this  shield,  an  inrush  .of  water  would  at  once  have  filled  the  tunnel 
to  the  soffit  of  the  roof. 

Subsequent  works  in  the  London  Clay,  however,  have  proved  that  there  is 
practically  no  risk  involved,  this  material  being  of  so  homogeneous  a  nature  that 
little  or  no  danger  is  to  be  apprehended  from  faults  and  a  consequent  inrush  of 
water. 

The  use  of  hand-worked  screw  jacks  in  a  tunnel  of  such  small  size  is  probably 
as  economical  as  the  employment  of  hydraulic  or  other  rams,  in  view  of  the 
obstruction  caused  by  pressure  pipes  and  the  like,  and  their  greater  liability  to 
break  down,  or  get  damaged,  in  the  conditions  in  which  tunnelling  works  are 
carried  on. 

Indeed  Mr.  Greathead,  as  late  as  1895,  gave  it  as  his  opinion  that  for  a  tunnel 
in  clay  of  similar  size  as  the  Thames  Subway  hand-worked  screws  had  a  decided 
advantage  over  hydraulic  rams  on  the  score  of  simplicity. 

On  the  other  hand,  it  is  not  easy  to  manipulate  at  the  same  time  say  three 
adjoining  screws  by  hand,  whereas  it  is  perfectly  easy,  with  hydraulic  rams — 
properly  connected — to  move  simultaneously,  several  or  all  at  a  time. 

As  stated  above,  cement  grout  was  injected  behind  the  iron  lining  of  the  tunnel 
by  means  of  a  hand  syringe,  but  owing  to  the  necessity  of  making  the  grout  suffi- 
ciently fluid  to  pass  through  the  syringe,  and  the  limited  amount  of  pressure  that 
could  be  applied,  the  making  of  a  complete  envelope  of  cement  round  the  tunnel 
was  not  satisfactorily  accomplished. 


The  Broadway  Tunnel 

At  the  same  time  that  the  Tower  Subway  was  projected  by  Barlow  in  England, 
a  Mr.  Beach  in  the  United  States  was  preparing  a  shield  modelled  on  Barlow's 
design  of  1864,  for  a  pneumatic  subway  under  Broadway.  This  subway,  8  feet  in 
diameter,  was  driven  through  loose  sandy  soil,  being  lined  with  brick  in  cement. 

The  shield  used  is  shown  on  Fig.  10,  which  is  a  longitudinal  section  of  the 
machine.  The  frame  of  the  shield  is  formed  of  a  heavy  timber  ring  A,  fronted 
with  a  cast-iron  cutting  edge  D,  and  having  at  the  rear  a  wrought-iron  forged  ring 
B,  to  take  and  distribute  the  thrust  of  the  rams  C,  C.  Instead  of  a  rectangular 
framing  as  in  Barlow's  shield,  the  face  of  the  excavation  is  supported  by  iron 
shelves  E,  E,  bevelled  off  on  the  front  edge.  To  the  rear  end  of  the  frame  A  is 
attached  a  flexible  cylinder  or  tail  of  steel  G,  which  overlaps  the  masonry  of  the 
tunnel  already  constructed. 


THE    SHIELD:    ITS    EARLY    HISTORY,    1818    TO    1880 

The  hydraulic  rams  C,  C,  are  operated  by  a  hand  pump  (not  shown)  with 
which  they  are  connected  by  pressure  pipes  passing  round  the  shield  inside  the 
frame  A.  These  rams  bear  on  the  masonry  of  the  tunnel  already  constructed,  and 
their  thrust  behind  is  distributed  by  bearing  blocks  of  wood  F. 

These  were  the  first  hydraulic  rams  used  in  a  shield. 

The  shield  worked  in  a  satisfactory  manner,  the  modus  operandi  being  to  drive 
the  shield  into  the  face  of  the  material  to  be  tunnelled  (the  total  pressure  obtained 
by  the  pump  being  about  120  tons),  to  the  extent  of  the  stroke  of  the  pumps  ;  the 
shelves  in  the  front,  while  they  entered  the  face,  serving  to  support  it. 


FIG.   10.     BROADWAY  SHIELD,  NEW  YORK. 
Beach's  Shield. 


When  the  shield  was  driven  forward  as  far  as  possible,  the  loose  soil  was 
removed  from  the  shelves,  at  the  same  time  that  the  permanent  masonry  lining  was 
built  up  beneath  the  shelter  of  the  tail  plate  of  the  shield. 

The  principal  defect  which  the  drawings  of  this  shield  show  is  a  lack  of  vertical 
stiffening,  there  being  no  vertical  plates  similar  to  the  horizontal  shelves  E,  E, 
Later  experience  has  shown  that  all  shields  unless  very  stiffly  braced  do,  after 
a  time,  spread,  so  that  the  horizontal  diameter  increases  while  the  vertical 
diminishes. 

15 


TUNNEL    SHIELDS 

This  work  was  carried  out  in  1 869-70, *  and  in  1872  a  similar  machine  was  used 
in  the  construction  of  a  subway  in  Cincinnati,  another  also  in  a  short  length  of  the 
lake  tunnel  at  the  same  place,  and  a  third  at  Cleveland,  Ohio.  The  tunnel  shield 
under  the  lake,  however,  was  only  used  for  a  length  of  about  140  feet,  where  the 
material  met  with  was  "  clay  "  of  such  fluidity  that  it  ran  in  at  the  face  faster  than 
the  permanent  brickwork  of  the  tunnel  could  be  put  in.  Here,  as  has  almost 
always  in  some  degree  been  the  case  with  a  masonry  tunnel  built  with  a  shield,  the 
brickwork  cracked  owing  to  the  advance  of  the  shield  allowing  no  time  for  setting. 
These  cracks  were  made  good  by  fixing  cast  iron  rings  or  "  tubbing  "  in  segments 
round  the  inside  of  the  brickwork  where  they  occurred. 

With  the  completion  of  these  tunnels  in  England  and  America,  the  use  of  shields 
in  similar  work  might  have  been  expected  to  become  general,  but  no  further  work 
was  carried  out  in  this  manner  until  the  City  and  South  London  Railway,  with  Mr. 
Greathead  as  engineer,  was  commenced  in  1886,  nor  with  one  exception  was  similar 
work  even  projected  before  that  year,  and  then  by  the  same  engineer. 

The  Woolwich  Shield,  1876 

.j 

In  the  year  1874  2  the  late  Mr.  Greathead  designed  and  constructed  for  a  pro- 
posed circular  iron  tunnel  under  the  Thames  at  Woolwich  a  shield  combining  in 
one  machine  the  water  or  fountain  trap,  more  recently  so  successfully  used,  and 
the  ordinary  a'rlock. 

For  reasons  unconnected  with  the  engineering  features  of  the  tunnel,  the  shield 
though  built  was  never  used,  and,  an  abortive  attempt  having  been  made  to  carry 
out  the  work  in  the  usual  manner  and  without  compressed  air,  the  whole  project 
was  abandoned. 

The  shield,  although  never  put  to  the  actual  test  of  working,  is  interesting  as 
being  the  first  one  built  with  a  water  seal,  and  also  the  first  in  which  it  was  proposed 
to  work  by  means  of  compressed  air,  thus  anticipating  by  five  years  its  actual  use 
at  Antwerp  and  at  the  Hudson  River  tunnel. 

Fig.  11  gives  a  diagrammatic  section  of  this  shield. 

The  cutting  edge  is  shown  of  considerable  length,  probably  to  afford  as  much 
protection  as  possible  to  the  miners  when  the  character  of  the  ground  might  permit 
of  work  being  carried  on  in  front  of  the  diaphragm.  This  latter  A,  extended  over 
the  upper  half  of  the  shield  only,  but  behind  this  front  diaphragm  was  placed  another, 
B,  which  closed  the  shield  completely,  an  airtight  door  C  opening  forwards  being 
fixed  in  its  upper  half.  Behind  this  diaphragm,  which  formed  the  front  of  it,  was 
placed  an  airlock,  having  its  outer  door  D  also  in  the  upper  half  of  the  shield. 

Behind  this  airlock  the  skin  of  the  shield  extended  sufficiently  far  to  accom- 
modate the  hydraulic  rams  E,  E,  and  to  overlap  the  tunnel  already  completed. 

The  proposed  method  of  working  was  apparently  to  keep  the  air  in  the  space 
in  front  of  the  diaphragm  B  at  the  required  pressure  to  dry  the  face  and  to  allow 
the  miners  to  work  under  cover  of  the  projecting  cutting  edge,  whence  they  could 
retreat  under  the  front  diaphragm  in  case  of  the  face  coming  in.  Mr.  Greathead's 
(Proc.  Inst.  C.E.,  vol.  cxxiii.  p.  66)  design  in  making  the  water  trap  between  the 
diaphragm  A  and  B  was  of  course  to  prevent  the  escape  of  any  large  volume  of  air 

1  It  was  opened  to  the  public  on  February  26,  1870,  see  Scientific  American,  March  5,  1870. 

2  Mr.  Greathead  patented  a  shield  for  working  in  loose  water-bearing  strata,  No.   173, 
of  1874. 

16 


THE    SHIELD:    ITS    EARLY    HISTORY,    1818    TO    1880 

in  case  of  a  "  blow."  As  designed,  however,  it  may  be  doubted  whether  even  the 
smallest  "  blow,"  if  sudden,  would  not  have  reduced  the  pressure  to  the  normal 
before  the  trap  came  into  action,  owing  to  the  smallness  of  the  air  chamber.  It  is 
essential  for  safe  tunnelling  in  compressed  air  to  have  a  large  volume  of  air  to  draw 
upon  in  the  event  of  a  "  blow  "  in  the  face. 

If  men  Were  working  between  the  two  diaphragms  with  the  front  door  of 
the  airlock  closed  when  a  blow  occurred,  their  chance  of  escape  would  be  small, 
as  the  air  pressure  would  almost  certainly  drop  so  rapidly  that  the  water  would 
rise  nearly  to  the  top  of  the  shield  and  prevent  the  opening  of  the  door  in  the 
diaphragm  B. 

A  similar  arrangement  of  locks  was  designed  and  made  for  the  Blackwall 
shield,  the  idea  being  in  that  case  to  provide  different  air  pressures  in  the  different 


Lock 


B 

Wafer  Level. 


FIG.  11.     WOOLWICH  SUBWAY  UNDER  THE  THAMES. 
Shield  designed  by  Greathead,  but  not  used. 

levels  of  the  shield,  but  it  was  not  used,  nor  so  far  as  the  author  is  aware  have 
differential  pressures  in  the  same  tunnel  been  tried  anywhere. 

As  stated  above,  however,  the  shield,  though  built,  was  never  used,  and  is  only 
figured  in  this  place  as  forming  an  interesting  advance  in  the  development  of  the 
simple  Greathead  shield  into  the  more  complicated  machine  for  subaqueous  work.1 

Of  equal  interest  is  the  mechanical  erector  designed  by  Mr.  Greathead  for  this 
tunnel,  and  which  has  formed  the  model  for  subsequent  machines  (see  Figs.  12 
and  13). 

.Cast  iron  as  a  permanent  tunnel  lining  was  for  the  second  time  used  at  Antwerp 
in  1879,  by  Mr.  Hersent,  the  contractor  for  the  extensive  dock  works  in  the  Scheldt. 
He  used  it  in  a  small  adit  (Fig.  14)  which,  however,  is  only  important  because  it 
was  the  first  tunnel  constructed  under  compressed  air.  The  peculiar  shape  of  the 

1  The  first  shield  actually  worked  with  a  water  seal  or  trap  was  that  used  in  the  tunnel  under 
the  Mersey  in  1889  (see  chapter  vii.). 

17  c 


FIG.   12.     WOOLWICH  SUBWAY  UNDER  THE  THAMES. 
Mechanical  Erector  for  putting  the   Cast-iron  Tunnel   Segments  in   position.       Sectional  Elevation. 


rm      rm     rfTi 


FIG.   13.     WOOLWICH  SUBWAY  UNDER  THE- THAMES. 
Mechanical  Erector  for  putting  the  Cast-Iron  Tunnel  Segments  in  position.     Sectional  Plan. 

18 


THE    SHIELD  :    ITS    EARLY    HISTORY,    1818    TO    1880 

tunnel  suggests  that  Mr.  Hersent  was  endeavouring  to  reproduce  in  cast  metal  the 
side  trees  and  head  trees  of  an  ordinary  miner's  heading. 

The  joints  are  of  a  peculiar  pattern,  a  groove  being  cut  in  the  flanges  of  the 
joints,  which  are  metal  to  metal,  to  receive  a  rope  of  tarred  hemp,  and  this  apparently 
formed  the  only  caulking  to  the  joint. 


GENERAL  OBSERVATIONS 

In  closing  this  introductory  chapter,  in  which  an  outline 'of  the  first  attempts 
at  tunnelling  under  shield  has  been  given,  and  before  proceeding  to  describe  modern 
shield  work,  which  may  be  said  to  commence  with  the  City  and  South  London 
Railway,  the  first  undertaking  (for  the  Tower  subway,  like  the  Broadway  tunnel, 
can  be  regarded  as  little  more  than  an  experiment)  in  which  the  new  system  was 
tried  on  a  large  scale  :  a  few  lines  may  be  given  to  the  consideration  of  the  vexed 
question  of  priority  of  invention  as  among  the  several  pioneers  in  modern  shield 
work. 

It  is  often  the  case  that  when  the  time  is  ripe  for  the  effective  use  of  a  new  idea, 
that  idea  occurs  to  several  independent  workers    at    the    same    time.      The  new 
necessity  arises,  a  new  departure  is  possible,  and 
men   whose    professional   studies  have    been   in 
that    direction,    may,    and    do,    arrive   at    very 
similar    discoveries,    in    complete    ignorance   of 
others'  work  on  the  same  lines. 

It  is  hardly  just  to  say  that  in  such  cases 
the  original  inventor  is  he  whose  name  appears 
first  in  the  lists  of  the  Patent  Office,  even  if  his 
idea  has  not  passed  beyond  the  paper  stage  ;  still 
less  is  it  fair  to  urge  that  others  who  subse- 
quently have  put  into  practice  similar  ideas, 
have  necessarily  borrowed  from  the  first.  Nor, 
when  a  new  machine  has  been  actually  con- 
structed and  put  to  work,  is  it  easy  for  the 
historian  to  determine  how  much  of  it  is  the 
original  inventor's  and  how  much  the  result  of  FlG  14 
many  minds  working  on  his  original  idea. 

There  is   no  oblivion  more    complete  than 

that  which  buries  a  patented  invention 1  which  for  any  reason  remains  untested 
by  actual  work,  and  consequently  there  is  hardly  any  charge  more  difficult  to  sub- 
stantiate against  an  inventor  than  that  of  conscious  plagiarism,  or,  one  may  even 
say,  one  which  in  general  it  is  more  unfair  to  make,  and  this  is  exemplified  in  the 
case  of  the  machines  under  consideration,  for  Greathead,  though  closely  associated 
with  Barlow,  remained  in  ignorance  for  nearly  thirty  years  of  the  latter's  patent 
of  1868. 

Three  men,  Barlow  and  Greathead  in  England,  and  Beach  in  America,  were 
undoubtedly  working  independently  at  the  shield  problem  in  the  late  sixties  (and 
very  likely  earlier). 

1  As  for  instance  Morton's  provisional  patent  of  1866,  above  quoted,  which  has  remained 
unknown  apparently  until  the  author  chanced  upon  it  in  search  of  information  for  purposes, 
of  this  chapter. 

IQ 


SMALL  TUNNEL  AT  ANTWERP.. 
Cross  Section  of  Cast-iron  Lining. 


TUNNEL    SHIELDS 

Of  these  men,  Barlow  was  certainly  the  first  to  patent,  in  1864,  a  shield  capable 
of  motion  in  one  piece,  and  surrounded  by  a  thin  cylinder  of  iron  within  which  he 
proposed  to  build  in  successive  rings  a  cast-iron  tunnel,  which  tunnel  he  proposed 
to  make  solid  by  injecting  grout,  how  he  did  not  say,  behind  the  cast-iron  lining  to 
fill  the  annular  space  left  by  the  advance  of  the  shield. 

This  was  in  1864,  and  in  1868  he  provisionally  patented  a  shield  having  near 
the  cutting  edge  a  transverse  partition  or  diaphragm. 

Neither  of  these  designs  took  practical  form,  and  in  1869  Greathead  in  England 
and  Beach  in  New  York  actually  built  and  used  shields  having  many  features  in 
common  with  Barlow's  patents  but  differing  from  each  other  in  details. 

Speaking  generally,  Beach's  shield  resembled  the  Barlow  patent  of  1864,  and 
Greathead's  the  provisional  patent  of  1868. 

Beach  states  that  he  first  designed  his  shield  in  1865,  and  that  he  in  1868 
actually  constructed  and  tried  an  experimental  model  3  feet  in  diameter.  He  does 
not  appear  to  have  been  aware  of  the  existence  of  Barlow's  shield,  though  the 
Tower  Subway  shield,  which  was  Greathead's,  not  Barlow's,  was  known  to  him. 

That  Greathead,  whose  shield  was  first  used  in  the  Tower  Subway  by  him 
acting  as  contractor  under  Barlow,  knew  of  the  first  Barlow  patent  is  certain,  but 
there  is  on  record  his  own  statement  that  until  the  fact  was  mentioned  in  the  dis- 
cussion at  the  Institution  of  Civil  Engineers  on  his  paper  on  the  "  City  and  South 
London  Railway  "  in  1895  1  he  was  unaware  of  the  existence  of  Barlow's  provisional 
patent  of  1868,  which  his  own  shield  most  resembles. 

That  in  the  construction  of  the  Tower  Subway,  a  shield  of  the  Greathead  model 
and  not  the  original  Barlow  design  was  employed,  would  indicate  either  that  Mr. 
Barlow  considered  the  Greathead  shield  as  constructed  an  advance  on  his  own,  or 
else,  as  is  probable  was  the  case,  that  the  whole  direction  and  management  of  the 
new  system  of  tunnelling  was  in  the  hands  of  Mr.  Greathead. 

The  exact  apportionment  of  the  credit  of  the  invention  between  these  two 
men  will  be  decided  by  each  reader  according  as  he  may  consider  the  original 
inventor  of  a  new  mechanism,  or  the  man  who  applies  it  to  practical  use,  the  more 
deserving  of  credit. 

It  has  been  said  above  that  the  idea  of  the  modern  shield  was  first  published 
by  Barlow  in  1864,  and  that  the  first  shields  actually  used  in  tunnel  work  were 
employed  practically  simultaneously  by  Greathead  in  England  and  Beach  in  New 
York  in  1869,  and  so  far  it  is  an  arguable  point  to  whom  credit  should  be  ascribed 
for  initiating  a  new  departure  in  tunnel  work.  But  in  the  subsequent  development 
of  the  shield  system  of  tunnelling  the  part  taken  by  Mr.  Greathead 2  has  connected 

1  Proc.  Inst.  C.E.,  vol.  cxxiii.  p.    110. 

2  James  Henry  Greathead,  the  engineer  to  whose  energy  and  perseverance  the  tunnelling 
system  described  in  this  book  is  mainly  due,  was  born  in  South  Africa  in  1844,  and  died  in 
London  in  1896. 

His  successful  construction  of  the  Tower  Subway,  when  as  contractor  he  carried  out  the 
work  for  Mr.  Barlow,  when  only  in  his  twenty-sixth  year,  apparently  determined  the  direction 
of  his  professional  energies  :  and  in  the  years  between  the  completion  of  that  work  and  the 
commencement  of  the  City  and  South  London  Railway  scheme  in  1884,  he  designed  and 
patented  various  appliances  for  tunnelling  in  water-bearing  strata  (Patents  No.  1738  of  1874,  and 
5665  of  1884),  and  later  in  1886  he  patented  also  (No.  5221  of  1886),  his  system  of  grouting 
by  means  of  compressed  air,  which  perhaps  more  than  any  other  invention  has  proved  indis- 
pensable in  all  recent  tunnel  work.  In  1874  he  designed  and  constructed  for  a  proposed  tunnel 
under  the  Thames  at  Woolwich  (which  for  reasons  unconnected  .with  the  engineering  features 
of  the  work  was  never  made),  a  shield  embodying  the  main  features  of  the  now  well  known  trap 

2O 


THE    SHIELD  :    ITS    EARLY    HISTORY,    1818    TO    1880 

his  name  more  than  that  of  any  other  man  with  this  branch  of  engineering,  which, 
next  to  the  construction  of  large  span  bridges,  is  the  distinctive  feature  of 
constructive  engineering  in  the  last  twenty  years  of  the  nineteenth  century. 

or  water-seal  shields  (see  Fig.  11),  and  designed  also  an  hydraulic  erector  which  is  the  original 
of  the  numerous  segment  erectors  which  have  been  made  since  (see  Figs.  12  and  13). 

In  1884  he  became  associated  with  the  City  and  South  London  Railway,  or  as  it  was  then 
styled,  the  City  and  Southwark  Subway,  on  which  his  system  of  shield  tunnel  was  for  the  first 
time  employed  on  a  large  scale.  The  satisfactory  completion  of  this  work  established  him  in 
the  first  rank  of  his  profession,  and  from  that  time  onward  to  his  death  he  was  employed  either 
as  engineer  or  as  consulting  engineer  in  every  important  tunnel  work. 

The  Hudson  Tunnel,  the  Blackwall  Tunnel,  the  Waterloo  and  City,  and  the  Central  Lon- 
don Railways  are  among  the  tunnels  with  which  he  was  connected,  while  among  other  works 
with  which  he  was  connected  at  the  time  of  his  death  was  the  Liverpool  Overhead  Railway,  of 
which  he  was  joint  engineer  with  Sir  Douglas  Fox. 

His  professional  qualities  can  be  estimated  from  the  above  bare  statement  of  the  under- 
takings he  assisted  in  :  his  personal  qualities  gained  him  the  esteem  of  all  who  had  dealings 
with  him,  while  his  unvarying  kindness  and  consideration  secured  him  the  warm  regard  of 
those  who  like  the  author  had  the  good  fortune  to  work  under  him. 


21 


THE   USE   OF   COMPRESSED   AIR   IN   ENGINEERING  WORK  :     ITS 
EARLY  HISTORY  :    AND  SOME  NOTES  ON  CAISSON  SICKNESS 

COCHRANE'S  PATENT,  1830 — DESCRIPTION  OF  AN  ORDINARY  AIRLOCK — COMPRESSED  AIR  USED 
AT  CHALONNES,  FRANCE,  1839 — AND  AT  DOTJCHY,  FRANCE,  1846 — POTTS'  VACUUM  SYSTEM, 
1850 — ROCHESTER  BRIDGE,  1851 — ANTWERP  TUNNEL,  1879 — HUDSON  RIVER  TUNNEL, 
1879 — JAMINET'S  NOTES  ON  ST.  Louis  BRIDGE,  1868 — BROOKLYN  BRIDGE,  1871 — SMITH'S 
PROPOSAL  FOR  A  MEDICAL  LOCK,  1871 — MOIR'S  LOCK  AT  HUDSON  TUNNEL,  1879 — CAISSON 
SICKNESS — CONDITIONS  OF  WORK  IN  COMPRESSED  AIR — REGULATIONS  FOR  CONTROLLING 
MEN — EFFECT  OF  IMPURE  AIR — CLAUSES  OF  SPECIFICATION  REGULATING  WORK  IN 
COMPRESSED  AIR — EXPERIMENT  IN  PURIFYING  AIR 

Historical  Notes  on  the  Use  of   Compressed  Air  in  Engineering  Works  1 

THE  fact  that  by  means  of  diving  bells  it  is  possible  for  human  beings  to  work 
in,  and  remain  for  some  time  under  water  in  an  atmosphere  the  pressure  of 
which  exceeded  the  normal  by  the  weight  of  the  water  above  it  was  known  in  the 
early  years  of  the  sixteenth  century,  and  some  use  was  made  of  the  system  ;  but 
the  absence  of  any  mechanism  for  renewing  the  air  prevented  any  prolonged  immer- 
sion, as  the  bell  or  working  chamber  had  necessarily  to  be  brought  to  the  surface  at 
short  intervals  on  account  of  the  vitiation  of  the  atmosphere. 

In  1664,  a  Dr.  Henshaw,  an  Englishman,  proposed  to  treat  certain  diseases 
by  immersing  the  patient  in  an  atmosphere  artificially  compressed,  or  exhausted, 
in  an  hermetically  sealed  chamber,  the  pressure  in  which  could  be  regulated  by 
means  of  bellows. 

Nothing  is  known  of  any  practical  trial  of  this  system  until  many  years  after, 
but  in  1830-40  a  compressed  air  treatment  for  pulmonary  diseases  was  practised 
in  France.2 

These  "  air-baths,"  as  they  were  called,  were  not  used  at  high  pressure  ;  about 
10  pounds  per  square  inch  above  the  normal  being  the  maximum  :  or  say  two- 
thirds  of  an  atmosphere. 

In  1721  Dr.  Halley  described  to  the  Royal  Society  an  arrangement  he  had 
made  whereby  fresh  air  could  be  supplied  to  diving  bells  by  means  of  weighted 

1  Much  useful  information  on  the  medical  aspects  of  compressed  air  work  is  contained 
in  : — 

A.  H.  Smith's  Compressed  Air,  Detroit,  U.S.A.,  1886. 

Jaminet's  Physical  Effects  of  Compressed  Air,  St.  Louis,  U.S.A.,  1871. 

E.  H.  Snell's  Compressed  Air  Illness,  London,  1896. 

Paul  Bert's  La  Pression  Barometrique,  Paris,  1878. 

Macmorran's  Notes  on  Caisson  Disease  (privately  printed),  London,  1901. 

2  Academic  des  Sciences,  vol.   xiii.,  contains  Mr.   Triger's   '-'  Memoirs  sur  un  Appareil  a 
air  comprime/' 

22 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

barrels,  which  enabled  the  workmen  employed,  in  them  to  remain  under  water  for 
long  periods,  but  it  was  not  until  1778  that  the  famous  Smeaton,  in  repairing  the 
foundations  of  the  bridge  over  the  River  Tyne  at  Hexham,  used  for  the  first  time 
a  pump  for  injecting  fresh  air  into  the  diving  bell  or  box  employed  there. 

In  1820  in  connexion  with  some  submarine  work  at  Howth,  near  Dublin,  a 
Russian  physician  named  Hamel  made  some  observations  on  the  effect  of  com- 
pressed air  not  only  on  the  workmen,  but  on  himself. 

He  described  his  sensations  on  going  down  in  a  diving  bell,  and  apparently 
was  more  fortunate  than  most  people  are,  for  he  states  that  "at  15  or  16  feet  (deep) 
or  about  with  7  pounds  pressure  there  was  a  noise  in  the  ears  like  an  explosion, 
followed  by  entire  relief  from  the  pain  "  1  experienced  in  first  going  down.  He 
makes  the  remark  that  one  of  the  workmen  became  so  accustomed  to  the  air  of 
the  bell  as  to  be  uncomfortable  under  the  usual  atmospheric  pressure. 

In  1826  Dr.  Colladon  also  published  some  observations  on  the  same  subject, 
and  he  is  said  to  have  recommended  to  Brunei,  two  years  later,  the  use  of  compressed 
air  in  his  Thames  Tunnel  undertaking,  not  only  as  a  source  of  mechanical  power, 
but  also  for  holding  up  the  working  face. 

It  was  in  1830,  however,  that  the  first  complete  scheme  of  mining  in  subaqueous 
or  in  water-bearing  material  by  the  aid  of  compressed  air  was  put  forward,  all  pre- 
vious engineering  applications  of  the  system  having  been  in  open  water  only.  In 
that  year,  Sir  Thomas  Cochrane,  known  by  the  courtesy  title  of  Lord  Cochrane, 
took  out  a  patent  (No.  6018  of  1830)  for  the  employment  of  compressed  air  in  shafts 
and  tunnels  in  water-bearing  material,  for  the  purpose  of  expelling  the  water 
from,  and  holding  up  the  face  of,  the  excavation,  and  the  wording  of  this  patent 
covers  all  the  essential  features  of  compressed  air  work  as  developed  since. 

Up  to  his  time,  the  various  compressed  air  appliances  in  use  do  not  appear  to 
have  been  provided  with  any  arrangement  for  easy  ingress  to  and  egress  from  the 
compressed  air  chamber,  of  workmen  and  material. 

As  the  use  of  the  airlock,  as  set  forth  in  his  patent,  has  always  been  a  feature 
of  later  work,  its  main  features  may  well  be  described  in  this  place. 

Since  the  employment  of  compressed  air  in  mining  work  necessitates  the  work- 
ing chamber  whether  that  be  a  shaft,  caisson,  or  tunnel,  being  practically  airtight, 
except  at  the  face  where  the  mining  work  is  to  be  done,  it  is  impossible  to  have  a 
working  door  giving  direct  access  to  the  chamber. 

The  difficulty  is  got  over  by  placing  a  small  chamber  or  "  lock,"  as  it  is  called, 
between  the  pressure  chamber  and  the  open  air.  This  lock  is  built  into  the  wall 
closing  the  pressure  chamber  and  is  provided  with  two  doors,  the  one  opening 
inward  into  the  compressed  air  space,  the  other  also  opening  inwards,  giving  access 
to  the  lock  from  the  open  air. 

When  the  working  chamber  is  filled  with  compressed  air  by  means  of  pumps 
the  inner  door  of  the  lock  is  of  course  kept  shut  by  the  pressure,  and  access  to  it 
from  outside  is  gained  by  entering  the  lock  and,  having  shut  the  outer  door,  opening 
a  valve  controlling  a  pipe  connexion  of  small  diameter  between  the  lock  and  the 
working  chamber.  This  allows  the  air  from  the  latter  to  enter  the  lock  until  the 
pressure  in  the  lock  is  equal  to  that  in  the  working  chamber,  which  is  then  entered 
by  the  inner  door  of  the  lock. 

The  open  air  is  reached  from  the  working  chamber  by  reversing  this  process  ; 

1  Smith's  Compressed  Air,  p.   3. 
23 


TUNNEL    SHIELDS 

the  outer  wall  of  the  lock  being  provided  with  a  pipe  by  which  the  compressed  air 
in  the  lock  can,  the  inner  door  being  shut,  be  allowed  to  escape  into  the  open 
air,  when  the  outer  door  can  be  opened. 

In  actual  work  the  airlock  takes  a  variety  of  forms,  which  will  be  noticed  later, 
but  the  airlock  and  bulkhead  shown  in  Figs.  15  and  16  embody  the  main  features 
of  all  the  locks  used  in  horizontal  tunnelling  work,  and  will  serve  as  an  example 
to  explain  the  general  principles  on  which  all  locks  are  built. 

In  this  case,  a  large  brick  sewer  had  been  constructed  by  ordinary  cut  and 
cover  methods  to  within  a  few  feet  of  a  water  way,  under  which,  at  a  depth  of  some 


Scale 


FIG  15.     TYPICAL  AIRLOCK  FOR  TUNNEL  WORK. 
Sectional  Plan  of  Lock  and  Bulkhead  used  in  the  Lea  Tunnel,  London,  1901. 


10  feet  below  the  bed  of  the  stream,  it  had  to  pass,  and  through  material  known 
to  be  water-logged. 

It  was  resolved  to  carry  out  the  work  by  shield  and  compressed  air,  and  to 
do  this  the  bulkhead  shown  in  the  figures  was  constructed  in  the  sewer  already 
built  at  some  little  distance  back  from  the  end  of  the  completed  length.  The  bulk- 
head to  close  up  the  tunnel  had  to  be  sufficiently  strong  to  resist  a  possible  pressure 
of  20  pounds  to  the  square  inch  on  its  inner  side,  and  of  course  to  be  absolutely 
airtight.  It  was  bonded  into  the  brick  sewer  already  built,  by  cutting  in  the 
latter  chases  18  inches  wide  into  which  the  new  brickwork  was  keyed.  The  wall 
was  in  all  8  feet  3  inches  thick,  a  9-inch  space  being  left  between  two  brick  walls, 

24 


THE    USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

into  which  grout  was  forced  under  pressure  through  the  grouting  pipes  shown  in 
Fig.  16.  A  rendering  of  neat  cement  1  inch  thick  on  the  inside  wall  of  the  bulk- 
head assisted  in  making  it  airtight. 

Into  this  bulkhead  was  built  a  cylindrical  airlock  of  steel  plates  £  inch  thick, 
17  feet  6  inches  long,  and  5  feet  10  inches  in  diameter,  so  placed  that  its  excess 
length  over  and  above  the  thickness  of  the  wall  projected  from  the  latter  on  the 
outside,  or  ordinary  atmosphere  side.  The  reason  of  this  was,  that  the  position 


FIG.   16.     TYPICAL  AIRLOCK  FOR  TUNNEL  WORK. 
End  Elevation  of  Airlock  and  Bulkhead  used  in  the  Lea  Tunnel,  London,  1901. 


ensured  the  outside  portion  of  the  lock  being  subjected  to  tensile  strain  only  from 
the  air  pressure. 

At  either  end  of  the  lock  is  a  door  opening  inwards,  that  is  towards  the  pressure 
chamber.  These  doors  were  fitted  accurately  to  their  frames,  the  bearing  surfaces 
being  provided  with  rubber  strips  to  ensure  an  airtight  fit. 

The  lock  was  fitted  with  four  "  mantaps  "  1  inch  in  diameter,  for  regulating 
the  supply  of  air  to  the  lock.  Two  of  these  were  outside  the  lock,  over  the  doors 

25 


TUNNEL    SHIELDS 

and  could  only  be  manipulated  from  the  pressure  chamber  or  from  the  outside 
respectively. 

The  other  two  were  within  the  lock,  and  like  the  others  controlled  pipes  com- 
municating with  the  two  sides  of  the  bulkhead,  the  taps  being  brought  together 
for  convenience  of  handling  by  one  man  within  the  lock. 

Four  larger  taps,  2|  inches  in  diameter,  known  as  "  muck-taps,"  were  also  pro- 
vided, and  could  be  worked  either  from  within  or  without  the  lock.  These  filled 
and  emptied  the  lock  much  more  rapidly  than  did  the  others,  and  were  used  for 
passing  material  through  it. 

A  pipe,  10  inches  in  diameter,  was  provided  through  the  bulkhead  for  the 
supply  of  air  to  the  compressed  air  chamber.  This  pipe  conducted  the  air  from 
the  compressors  through  the  bulkhead,  and  had  on  its  inner  end  a  flap  valve,  so 
that  in  the  event  of  the  pipe  being  broken  outside,  the  pressure  in  the  chamber 
would  not  be  lost  by  the  air  escaping  back  through  the  pipe. 

A  waste  air  pipe  5  inches  in  diameter  was  provided  to  allow  of  air  being  drawn 
off  from  the  pressure  chamber,  in  the  event  of  the  working  face  proving  so  solid  as 
to  necessitate,  for  reasons  of  hygiene,  changing  the  air  in  which  the  miners  were 
working. 

The  above  form  all  the  essential  features  of  a  bulkhead  and  airlock,  which 
compressed  air  demands,  but  there  are  of  course  many  auxiliary  fittings  connected 
with  the  general  tunnel  work  which  are  also  provided  when  a  bulkhead  is  put  up. 
Such  are,  as  in  the  figure  under  consideration,  hydraulic  pipes  for  working  the 
shield,  blow-out  pipes  for  removing  water  from  the  invert,  air  pipe  for  grouting, 
pipes  for  electric  light  wires,  etc. 

It  may  be  observed  here  that  all  service  pipes,  of  whatever  nature,  inserted  in 
the  bulkhead,  should  be  of  a  diameter  to  meet  every  possible  contingency  which 
may  arise.  The  extra  cost  of  placing,  in  the  first  instance,  a  somewhat  larger  pipe 
than  the  probable  conditions  of  the  work  may  require  is  small,  and  is  good  insurance 
against  the  cost  of  making  an  enlargement  later. 

In  Cochrane's  patent  he  provides  first  for  an  airlock  for  shaft  sinking,  and 
goes  on  to  say  that,  the  shaft  being  sunk,  one  or  more  locks  may  be  provided  in  the 
heading  or  tunnel  driven  from  it,  so  that  the  men  only  who  are  working  at  the  face 
of  the  excavation,  may  have  to  endure  the  maximum  pressure  required  to  keep  back 
the  water  there,  and  the  remaining  operations  in  the  rear  carried  out  under  a  less 
pressure. 

This  idea  of  differential  pressures  is  not  now  used  :  it  was,  however,  a  feature 
of  the  Blackwall  Footway  Tunnel  as  proposed  in  1888,  and  the  shield  actually  con- 
structed for  the  tunnel  at  the  same  place  in  1891  was  made  with  locks  and  chambers 
in  which  different  pressures  could  be  maintained  to  suit  their  different  levels  ;  or 
to  keep  the  pressure  of  the  work  chamber  in  front  higher  than  that  in  the  tunnel 
behind. 

Cochrane's  specification  is  accompanied  by  sketches  of  an  airlock,  and  a 
sectional  elevation  of  a  shaft  and  tunnel  in  course  of  construction  by  his  compressed 
air  method  (see  Figs.  17  and  18). 

His  drawing  shows  also  a  proposed  method  of  conveying  spoil  from  the  com- 
pressed air  workings  underground  to  the  normal  atmosphere  at  the  surface  by 
means  of  a  water  column,  and  a  bucket  and  chain  dredger. 

This  has  not  been  used  in  any  work  to  the  author's  knowledge  :  the  earliest 
shafts  or  caissons  sunk  by  means  of  compressed  air  having  the  locks  for 

26 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

ingress  and  egress  so  arranged  that  material  and  spoil  can  be  passed  through 
them. 

Mr.  Greathead  makes  the  remark  that  Cochrane  does  not  appear  to  have  con- 
sidered the  other  necessary  appliances  for  tunnelling  in  loose  water-bearing  strata 
which  are  almost  as  necessary  as  the  airlock  ;  indeed,  he  suggests  that  the  inventor 
did  not  contemplate  the  use  of  compressed  air  in  tunnels  except  in  materials 
impervious,  or  nearly  impervious,  to  water.1  The  wording  of  the  specification  hardly 
justifies  this  assumption,  and  perhaps  the  best  explanation  of  the  absence  of  any 
reference  to  the  necessity  of  additional  appliances  for  supporting  the  face  is  that  it 
did  not  occur  to  Cochrane  that  air  pressure  alone  would  not  hold  up  the  face,  the 
head  of  water  being  less  at  the  crown  of  a  tunnel  than  at  the  invert,  and  that  con- 
sequently the  pressure  which  would  sustain  the  face  at  the  one  level  would  not 
balance  the  water  pressure  at  the  other. 

How  to  do  this,  however,  is  a  problem  the  satisfactory  solution  of  which  is  still 
wanted. 


/j—  or  CLruds  chcurubef 


to  retaJ-n  -sarnie,  /"LighJUj  condem&ecL  ^ 
ct-ir  irii  tfie  end. /irLrt:  of  thje  e  tcafcLtLon, 

FIG.   17.     COCHKAJSTE'S  SYSTEM  OF  TUNNELLING  BY  COMPRESSED  AIR. 
From  drawing  attached  to  Specification  of  Patent  No.   6018  of  1830. 

It  is  interesting  to  note  in  view  of  the  common  practice  of  to-day  in  sinking 
shafts  and  caissons  that  Sir  T.  Cochrane  distinctly  states  in  his  specification  that 
"  in  case  the  column  (or  shaft)  should  not  prove  heavy  enough  to  sink  by  its  own 
weight  when  filled  with  compressed  air  the  workmen  may  come  out  from  the  shaft, 
and  then  let  the  compressed  air  from  it  in  order  that  it  may  sink  by  means  of  its 
own  weight  :  the  upward  pressure  being  thus  removed." 

The  system  proposed  by  Lord  Cochrane  in  1830  was  not,  however,  put  to  actual 
use  until  1839,  when  a  French  engineer,  M.  Triger,  employed  it  to  sink  a  pit  shaft 
at  Chalonnes  on  the  Loire,  through  the  geologically  recent  water-bearing  strata 
forming  the  valley  of  that  river  to  reach  the  coal-bearing  strata  below,  which 
previously  had  been  considered  inaccessible  for  mining  purposes. 

M.  Triger  gives  2  in  his  communication  describing  the  operations  to  the 
Academic  des  Sciences,  which  paper  is  the  authority  for  the  facts  given  below,  but 
little  detail  as  to  the  actual  carrying  out  of  the  work  beyond  stating  that  having 
sunk  the  tube  or  shaft,  which  was  some  3  feet  4  inches  in  diameter,  to  a  depth  of 

1  Proc.  Inst.   C.E.,  vol.  cxxiii.  p.   58. 

2  Academic  des  Sciences,    1841,  vol.   xiii.    "  Memoire  sur  un  Appareil  a  air  comprime." 
This  is  a  very  interesting  and  lively  description  of  compressed  air  work  of  the  time.     Some  of 
it  is  quoted  in  Proc.  Inst.  C.E.,  vol.  x.  p.  361. 

27 


TUNNEL    SHIELDS 


IPLan-  of  CLfificLraJju-s  for 
ecL  air  i 


CLnJJp  Chamber 


~Yer 


about  60  feet,  no  further  progress  could  be  made  by  loading  it,  or  striking  it  with 
a  pile  driver,  but  that  by  the  use  of  compressed  air  he  was  enabled  to  sink  the  tube 
completely  through  the  water-bearing  strata,  and  enter  the  impervious  clay  beds 
below  them. 

The  pressure  employed  seems  to  have  reached  two  atmospheres  above  the 
normal,  and  the  mechanical  arrangements  resembled  in  essentials  those  described 
in  Cochrane's  specification  and  still  in  use  to-day. 

The  lock  used  was  provided  with  the  usual  doors  and  valves  for  ingress  and 
egress  ;  it  had  also  a  pressure  gauge,  and  what  is  more  remarkable,  a  safety  gauge. 
Through  the  lock  passed  an  air-supply  main,  and  a  'c  blow-out  "  pipe,  the  latter 
being  extended  into  the  water  at  the  bottom  of  the  tube  "  to  facilitate  the  exit  of 
the  water,  when  as  a  result  of  the  air  pressure,  this  water  must  be  forced  out  with 

more  rapidity  than  the  leaks  between  the 
bottom  of  the  pipe  and  the  ground  would 
allow  of." 

The  pumps  and  engine  for  the  supply 
of  air  were  of  a  make-shift  character,  but 
no  difficulty  seems  to  have  been  experi- 
enced in  maintaining  the  pressure  required. 
M.  Triger  records  indeed  several 
"  blows,"  or  escapes  of  air  caused  by  the 
pressure  of  air  within  the  tube  being  in 
excess  of  the  hydrostatic  head,  and  from 
his  observation  of  the  ebullition  produced 
in  the  surrounding  water  (for  the  shaft 
was  apparently  in  the  river  itself)  and  of 
the  periodicity  of  the  "  blows  "  he  deduces 
a  theory  as  to  the  causes  of  the  eruptions 
of  the  geysers  in  Iceland — a  curiously  in- 
genious suggestion. 

Another  point  noted  by  M.  Triger  was 
that  when  the  excavation  of  the  shaft  was 
at  such  a  depth  that  the  necessary  pressure 
of  air  could  barely  be  maintained,  and 
when  consequently  the  vertical  "  blow-out " 

pipe  worked  badly  owing  to  the  nearly  equal  pressure  of  the  column  of  water  in  it, 
and  of  the  compressed  air,  an  increased  discharge  of  water  could  be  got  by  put- 
ting a  tap  in  the  pipe  about  half  way  up,  through  which  compressed  air  could  be 
let  into  the  pipe,  to  blow  out  the  water  above  it. 

This  device,  used  often  since,  was  suggested  by  the  results  of  an  accidental 
blow  from  a  miner's  pick  which  knocked  a  hole  in  the  "  blow-out  "  pipe. 

The  explanation  given  by  M.  Triger  of  the  effect  of  such  a  hole,  namely  that 
the  mixture  of  air  and  water  so  produced  made  a  mixture  of  less  specific  gravity 
than  water  alone,  is  hardly  satisfactory. 

The  real  explanation  is  that  an  opening  having  been  made  in  the  blow  out  pipe 
at  a  level  where  the  head  of  water  is  one-half  of  that  for  which  the  air  pressure  at 
the  time  is  adjusted,  the  air  naturally  forces  out  the  water,  and  in  doing  so  acts  some- 
what in  the  manner  of  an  injector,  and  assists  in  drawing  up  the  column  of  water 
in  the  lower  portion  of  the  blow-out  pipe. 

28 


VaLre 


n 


•  Corer  Co  Sfiaft 


*  Ort_enJLnjs!>  to 
'    ShafL. 


FIG.  18. 


COCHRANE'S  SYSTEM  OF  TUNNELLING 

BY  COMPRESSED  AIR. 

From  drawing  attached  to  Specification  of 
Patent  No.   6018  of  1830. 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

The  effect  of  compressed  air  on  those  entering  it  is  described  at  some  length 
by  M.  Triger,  in  general  with  great  accuracy — and  accuracy  perhaps  the  less 
remarkable  as  the  work  carried  out  at  Chalonnes  was  the  first  opportunity  for 
observing  the  effect  of  continued  pressure  on  numerous  persons  at  the  same  time, 
and  under  similar  circumstances. 

The  sensation  of  pain  in  the  ears  on  changing  pressure  :  the  necessity  for  taking 
a  longer  period  in  entering  the  pressure  chamber  than  in  leaving  it  :  the  increased 
rapidity  of  combustion,  and  increase  of  temperature  with  increase  of  pressure  :  the 
condensation  of  the  moisture  in  the  atmosphere  when  the  pressure  drops  :  the  com- 
parative ease  with  which  a  man  can  climb  a  ladder,  and  the  impossibility  of  his 
whistling,  under  pressure  ;  are  all  set  forth  for  the  first  time,  with  vivacity  and 
detail :  and  though  some  of  the  statements  may  require  qualification,  the  description 
of  the  objective  features  of  compressed  air  work  is  singularly  clear  and  complete. 

Later  in  1845  M.  Triger  addressed  a  letter  to  Mr.  Arago,  which  was  read 
at  the  Academic  des  Sciences,1  to  the  effect  that  the  shaft  sunk  in  1841  was  still 
perfectly  sound  and  that  another  similar  one  was  in  hand. 

This  letter  concludes  by  recommending  the  use  of  compressed  air  in  tunnel 
construction. 

In  1846  a  M.  Blavier  in  a  communication  printed  in  the  Annales  des  Mines  2 
describes  some  operations  similar  to  those  carried  out  by  M.  Triger,  from  which 
in  fact,  they  were  copied,  at  Douchy  in  North-East  France. 

A  shaft  was  satisfactorily  sunk  there  by  means  of  compressed  air,  and  M. 
Blavier  records  that  a  pressure  of  two  atmospheres  above  the  normal  was 
employed. 

Compressed  air  sickness  (as  distinguished  from  the  inconveniences  felt  in  enter- 
ing and  leaving  the  pressure  chamber)  was  noted  there,  and  was  said  to  be  cured, 
even  after  lasting  some  hours,  by  rubbing  the  affected  part  with  alcohol. 

As  a  result  of  the  experience  gained  at  Douchy,  M.  Blavier  ventured  the 
opinion  that  a  depth  of  about  64  feet  below  water  level  was  the  limit  of  possible 
working. 

Neither  M.  Triger  nor  M.  Blavier  appear  to  have  had  their  attention  drawn 
to  the  share  that  length  of  immersion  has  in  producing  sickness,  except  that  M. 
Triger  notes  briefly  that  two  men,  after  seven  hours'  continuous  work  in  the  shaft, 
experienced  severe  pains  in  the  arms  and  knees,  coming  on  in  about  half  an  hour 
after  coming  out  of  the  pressure  chamber. 

But  at  Douchy  a  M.  Pol,  apparently  describing  the  same  work  on  which  M. 
Blavier  was  engaged,  goes  into  some  detail  as  to  the  effect  of  compressed  air  on  the 
health  of  the  men  employed,  and  among  other  conclusions  drawn  from  his  experi- 
ence, states  that  re-immersion  is  the  quickest  and  safest  means  of  restoration  of  a 
man  affected  by  "  bends  "  as  the  pains  in  the  joints  caused  by  compressed  air  are 
called. 

About  this  period  was  tried  in  England  and  in  Ireland  a  method  of  shaft 
sinking  by  exhausting  the  air 3  in  the  cylinder  or  shaft  to  be  sunk,  known  as  Dr. 
Pott's  vacuum  system.  Although  not  strictly  a  part  of  the  history  of  compressed 
air  work,  this  process  for  a  short  time  was  a  rival  to  the  ordinary  one,  and  should 
therefore  be  briefly  described. 

1  Academic  des  Sciences,  1845,  vol.  xx.  pp.  444-449. 

2  Annales  des  Mines,  vol.  ix.  p.  349. 

3  Proc.  Inst.  C.E.,  vol.  x.  pp.   356-366-367. 

29 


TUNNEL    SHIELDS 

The  manner  of  sinking  a  shaft  by  Pott's  method  consisted  in  attaching  to  the 
top  of  the  cylinder  to  be  sunk  by  an  airtight  joint,  a  box  or  lock  which  for  some 
reason  was  known  as  the  "  doctor,"  provided  with  two  doors,  the  one  communicating 
with  the  shaft  the  other  with  the  open  air.  When  the  doors  were  closed  an 
exhaust  pump  outside  the  "  doctor  "  was  set  to  work,  and  the  air  in  the  "  doctor  J> 
exhausted  as  far  as  possible.  Then  by  means  of  a  valve  controlled  from  outside 
a  pipe  connecting  the  doctor  with  the  inside  of  the  shaft  was  opened,  and  the  inrush 
of  air  into  the  empty  doctor  created  a  partial  vacuum  in  the  shaft.  This  had  the 
effect  of  drawing  in  the  soil  at  the  bottom  of  the  shaft,  and  so  permitting  it  to  sink. 

The  system  was  tried  in  1850  by  Mr.  W.  H.  Hemans  at  a  bridge  over  the 
Shannon,  but  was  abandoned  after  three  cylinders  of  10  feet  diameter  had  been 
sunk  ;  the  expense  being  too  great,  and  the  rate  of  progress  unsatisfactory. 

It  was  also  tried  at  Rochester1  in  1851,  but  pronounced  impracticable,  and 
indeed  it  is  not  easy  to  see  how  it  could  succeed  except  in  perfectly  uniform  fine 
gravel  or  silt.  If  boulders  were  met  with  the  suction  of  the  air  could  hardly  be 
effective. 

The  system,  however,  appears  to  have  been  dropped  altogether  about  1851. 

In  England  the  first  large  work  carried  out  by  means  of  compressed  air  was  the 
Chepstow  Viaduct 2  (1843-1851),  the  foundations  of  which  were  built  under  a  head 
of  water  of  70  feet.  But  the  best  description  of  compressed  air  work  while  the 
system  was  still  in  the  experimental  stage  is  to  be  found  in  Mr.  Hughes'  account 
of  the  rebuilding  of  Rochester  Bridge.3 

The  Rochester  Bridge 

In  1851  the  then  existing  bridge  over  the  Medway  connecting  Rochester  and 
Strood  having  become  inadequate  to  accommodate  the  increased  traffic  the  con- 
struction of  a  new  one  was  intrusted  to  Mr.  Cubitt,  whose  design  included  the 
placing  of  the  abutments  and  piers  of  the  bridge  on  cylindrical  piles  of  cast  iron 
filled  with  concrete  and  brickwork,  and  capped  by  cast-iron  bed  plates  on  which 
the  masonry  piers  were  to  be  built. 

Each  pier  was  supported  on  fourteen  piles  or  cylinders,  7  feet  in  diameter,  and 
going  down  from  40  to  60  feet  below  mean  high  water  :  the  piles  ultimately  resting 
on  the  chalk,,  the  beds  passed  through  in  sinking  consisting  of  soft  clay,  sand,  and 
gravel,  all  water-bearing. 

It  was  originally  intended  to  sink  the  cylinders  by  means  of  Dr.  Pott's  vacuum 
system  above  referred  to,  and  the  apparatus  was  actually  installed  at  the  pier 
nearest  to  Strood,  but  it  was  found  impossible  to  sink  the  piles  by  such  means 
through  a  thick  bed  of  rubble  met  with,  which  had  formed  part  of  the  foundations 
of  an  earlier  stone  bridge. 

The  vacuum  method  was  therefore  abandoned,  and  the  use  of  compressed  air,, 
which  it  was  in  the  knowledge  of  the  engineers  concerned  had  proved  successful  at 
Chalonnes,  was  resolved  on. 

The  general  arrangements  for  the  cylinder  sinking  were  as  follows  :  at  each 
pier,  or  abutment,  a  timber  stage  was  erected  large  enough  to  afford  a  working 
platform  around  the  piles,  and  provide  room  for  machinery,  etc.  The  cylinders 

1  Proc.  Inst.  C.E.,  vol.  x.  pp.   356,  367.  2  Ibid.  p.   367,  footnote. 

3  Ibid.  p.  353,  Hughes  "  On  the  Pneumatic  Method  adopted~in  Constructing  the  Founda- 
tions of  the  New  Bridge  across  the  Medway  at  Rochester." 

30 


OF  THE 

NIVER3ITY 

OF 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

were  then  sunk  as  far  as  possible  by  gravity,  and  pitched  in  their  proper  positions, 
a  sufficient  number  of  the  9  feet  lengths  of  which  each  cylinder  was  composed  being 
built  together  to  bring  the  top  well  above  water  level.  This  done,  to  the  top  flange 
of  the  uppermost  casting  was  fixed  a  wrought-iron  plate  through  which  passed 

Fvg.l. 

SECTIONAL    ELEVATION 


FIG.   19.     ROCHESTER  BRIDGE,  KENT,  1851. 
Method  of  Sinking  Cylinders  under  Compressed  Air.    Sectional  Elevation  of  Cylinder  fitted  with  Airlock. 

"  two  cast-iron   chambers  which  may  appropriately  be  called  airlocks  "  (the  first 
time  this  name  was  applied  to  them). 

These  chambers,  marked  E  E  in  Figs.  20  and  21,  were  over  6  feet  in  depth 
and  D-shaped  in  plan  and  provided  with  doors  F  F  in  the  top  and  on  the  flat  verti- 
cal face  for  communicating  respectively  with  the  outer  air  and  the  cylinder  below. 

31 


TUNNEL    SHIELDS 


B_ 


The  arrangement  of  the  doors  by  which  two  airlocks  of  sufficient  size  are  put 
on  one  cylinder  only  7  feet  in  diameter,  and  so  arranged  that  they  can  be  worked 
simultaneously  with  the  minimum  of  obstruction,  is  very  neat,  the  airlocks  being 
arranged  so  that  their  flat  vertical  sides  are  on  the  same  diameter  of  the  cylinder 
but  facing  in  opposite  directions. 

Within  the  cylinder  and  on  a  level 
with  the  lower  doors  of  the  airlocks  were 
fixed  two  small  cranes  G  G,  one  for  each 
airlock,  the  jib  of  each  one  having  a  sweep 
over  one-half  the  area  of  the  cylinder,  so 
that  when  the  lower  door  of  the  corre- 
sponding airlock  is  opened,  the  loaded 
buckets  of  excavation  brought  up  by  the 
crane  can  be  swung  into  the  locks.  This 
arrangement  of  cranes  would  hardly  be 
satisfactory  with  the  large  skips  or  buckets 
now  in  use,  but  no  doubt  it  answered  satis- 
factorily with  the  small  buckets  shown  on 
the  drawing. 

A  double  set  of  valves  were  provided, 
the  one  for  the  use  of  men  passing  through 
the  airlock  and  the  other  to  enable  the 
men  in  the  cylinder,  or  the  banksman  out- 
side, to  operate  the  lock  in  order  to  pass 
material  through  without  entering  the  lock 
itself. 

The  "  blow-out  "  pipe,  which  passed 
through  the  cylinder  at  a  point  below  the 
airlocks,  formed  a  syphon,  the  long  leg  of 
which  reached  the  bottom  of  the  cylinder, 
and  the  short  leg,  outside,  mean  water 
level. 

The  actual  column  of  pressure  there- 
fore in  the  pipe  was  that  due  to  the  differ- 
ence in  level  between  the  bottom  of  the 
pipe  inside  the  cylinder  and  the  actual 
water  level  outside,  less  the  power  of  suc- 
tion of  the  syphon. 

It  was  soon  found,  however,  that  the 
sudden  variations  of  air  pressure  made 
possible  by  leaving  the  "  blow-out  "  pipe 
free  to  act  at  all  times  produced  so  thick 
a  fog  in  the  cylinder  as  to  impede  the  work, 
and  to  remedy  this  a  valve  was  fitted  to 
its  lower  end  by  which  the  pipe  could  be 
opened  or  shut  as  required,  and  the  escape 

of  air  controlled,  by  the  men  working  at  the  bottom  of  the  cylinder.    This  appears 
to  be  the  first  recorded  case  in  which  this  control  was  provided. 

The  principal  and  grave  objection  to  the  arrangements  described  above  is 

32 


FIG.  20.     ROCHESTER  BRIDGE,  KENT,  1851. 

Sectional  Elevation  of  Cylinder  on  line  D  D, 
Fig.  21. 


33 


D 


that  by  fixing  the  airlocks  on  the  top  of  the  cylinder  it  was  necessary,  whenever 
the  cylinder  had  sunk  so  that  the  top  of  the  length  erected  approached  high-water 
level,  to  stop  the  work  of  excavation,  let  off  the  air  pressure,  and  remove  the  locks 
in  order  to  erect  on  the  length  of  cylinder  already  sunk,  more  sections  of  9  feet 
length,  on  the  top  of  which  the  locks  were  then  replaced. 

This  probably  was  not  necessary  more  than  once  in  sinking  each  cylinder  on 
this  particular  work,  but  even  on  this  assumption,  ninety  stoppages  of  work  would 
be  required  during  the  building  of  the  piers  and  abutments.1 

The  alternative  arrangement,  viz.,  to  erect  the  entire  length  of  cylinder  before 
commencing  work  under  air  pressure,  would  have  the  double  disadvantage  that  a 
cylinder  perhaps  60  feet  long  and  only  7  feet  in  diameter  would  be  difficult  to  pitch 
accurately  and  that,  as  the  depth  of  the  cylinder  necessary  at  each  point  could  not 
be  known  beforehand,  no  security  could  be  felt  that  the  length  actually  erected 
would  suffice. 

The  true  solution  of  the  difficulty  is  to  put  the  airlocks  inside  one  of  the  cylin- 
der segments,  and  so  allow  them  to  sink  with  the  cylinder,  until  this  has  reached 
its  foundation,  when  they  are  removed  once  and  for  all. 

Another  feature  in  the  method  of  sinking  employed  which  deserves  notice  is 
the  system  of  kentledge  adopted  (see  Figs.  19  and  22). 

Across  the  top  of  the  cylinder  were  laid  two  beams  or  yokes  H,  H  so  placed  as 
to  clear  the  upper  doors  of  the  airlocks.  These  beams,  about  18  feet  long,  overhung 
the  adjacent  piles  on  either  side  which  were  pitched  ready  for  sinking.  At  these 
extremities  were  fixed  pulley  blocks,  over  which  passed  chains,  each  chain 
being  fastened  at  one  end  to  the  cylinder  below  it,  and  at  the  other  to  a  segment  J 
of  the  smaller  cylinders,  6  feet  in  diameter,  to  be  used  later  in  the  abutments, 
which  thus  hung  within  the  larger  cylinder,  and  could  travel  up  and  down  in  it. 

The  segments  J,  J,  having  each  a  temporary  bottom  fitted,  could  be  loaded 
as  desired,  and  by  the  device  of  also  hanging  these  segments  to  two  bars  K,  K, 
which  could  in  turn  be  supported  by  pins  at  L,  L,  it  was  possible  at  any  time 
to  take  all  the  pressure  off  the  chains  slung  over  the  beams  H,  H. 

The  cylinders  were  lowered  by  excavating  for  a  depth  of  about  14 
inches  below  the  bottom  of  the  cylinder,  and  the  men  having  withdrawn,  drop- 
ping the  pressure  until  the  cylinder  sank,  exactly  in  the  manner  set  forth  by  Coch- 
rane  in  1830. 

The  work  was  carried  out  very  successfully,  and  with  very  great  economy  as 
compared  with  the  probable  cost  of  constructing  the  foundations  by  means  of  coffer 
dams. 

The  accounts  extant  of  this  undertaking  give  very  little  information  as  to  the 
effect  of  compressed  air  work  on  the  men  engaged  ;  it  is  known,  however,  that 
there  were  no  fatal  cases  of  illness.2 

The  operations  in  connexion  with  Rochester  Bridge  have  been  described  in 
some  detail  as  being  typical  of  a  large  number  of  other  cases,  mostly,  like  them,  of 
bridge  foundations  below  water  level.  Compressed  air  was  for  many  years  used 
only  for  shaft  or  caisson  sinking,  and  Cochrane's  idea  that  it  would  be  employed 
in  tunnel  work  remained  untested.  But  in  caisson  work  the  system  became  at 

1  It  was  not  until  1867,  in  the  construction  of  a  bridge  over  the  Garonne  at    Bordeaux 
that  the  airlock  was  made  in  the  shaft  itself,  so  that  the  lock  went  down  with  the  shaft.     See 
Annales  des  Fonts  et  Chaussees,  vol.  ii.  of  1867,  p.  27. 

2  Dublin  Journal  of  Medical  Science,  1863,  xxxvi.  pp.  312-318. 

34 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

once  of  universal  employment.  At  Chepstow  (1843-51),  Saltash 1  (1854-59), 
Szegedin  (1856), 2  Bordeaux3  (1859),4  Koffre  Ozzyat  (1859),  Argenteuil  (1859), 
Bayonne  (1862),  Londonderry  (1862),5  L'Orient  (1862),6  and  Nantes6  (1864),  the 
foundations  of  bridge  abutments  were  built  by  this  method,  and  by  the  early  sixties 
may  be  said  to  be  established  as  the  customary  method  of  carrying  such  works  in 
water-bearing  material. 

It  was  not  until  1879  that  the  system  was  tried  in  tunnel  work,  and  then, 
curiously  enough,  it  was  put  into  practice  simultaneously  at  Antwerp,  and  at  New 
York. 

In  the  former  place  Mr.  Hersent  was  carrying  out  some  extensive  dock  works 
and  river  walls,  the  latter  of  which  he  built  by  means  of  caissons  sunk  under  com- 
pressed air,  and  having  to  make,  in  order  to  provide  for  some  pumping  work,  a 
small  tunnel  or  adit  entirely  in  fine  silty  sand  and  water,  he  employed  compressed 
air  to  keep  up  the  face.  No  shield  was  used,  and  the  lining  of  the  tunnel  was  of 
cast  iron.  The  height  of  the  tunnel  was  barely  5  feet,  and  consequently  no  trouble 
was  experienced  by  the  difference  in  pressure  required  to  balance  the  water  at  the 
crown  and  at  the  invert. 

The  excavation  was  carried  forward  in  lengths  of  about  1  foot  8  inches,  corre- 
sponding to  the  width  of  the  cast-iron  rings  of  the  lining,  and  no  timbering  of  any 
kind  was  used. 

But  when  this  small  work  was  in  progress  in  Antwerp,  a  similar  tunnel,  but 
on  a  much  larger  scale,  was  commenced  in  New  York.  An  attempt  was  made  to 
build  a  brick  tunnel  under  the  Hudson  River  through  a  material  of  silt  or  mud  so 
fluid  that  it  would  flow  through  the  smallest  crevices  almost  like  water  :  and  the 
engineer  in  charge,  Mr.  Haskin,  employed  compressed  air  as  a  support  for  the  roof 
of  the  excavation  as  well  as  for  expelling  the  water.7 

The  undertaking  was  not  successful,  but  the  use  of  compressed  air  fully  justified 
Mr.  Haskin  in  using  it,  and  since  then  it  has  been  employed  in  all  similar  work. 

Before  going  into  the  details  of  this  and  other  tunnel  works  where  compressed 
air  has  been  used,  a  few  pages  may  be  given  to  the  consideration  of  the  peculiar 
form  of  sickness  produced  in  some  cases  by  working  in  compressed  air,  and  of  the 
methods  adopted  for  its  avoidance,  and,  when  it  occurs,  for  its  cure,  or  at  any  rate 
its  temporary  alleviation,  for  the  medical  treatment  of  the  disease  hardly  comes 
within  the  scope  of  this  work. 

Caisson  Disease 

In  these  earlier  undertakings  recorded  above  the  effect  of  compressed  air  work 
on  the  health  of  the  men  engaged  in  it  was  in  some  cases  noted,  especially  by  M. 
Triger  at  Chalonnes,  who  gives  inter  alia  a  very  vivacious  account  of  his  sensations 
on  being  suddenly  "  locked  out  "  from  a  pressure  of  two  atmospheres,  and  by 
Messrs.  Pol  and  Watelle,  who  in  1845  made  observations  on  the  miners  engaged 
under  M.  Blavier  at  Douchy,  but  naturally  in  the  records  extant  more  attention 
is  given  to  the  engineering  than  to  the  medical  features  of  the  work.  In  the  case 

1  Proc.  Inst.  C.E.,  vol.  xx.  p.  268. 

2  Annales  des  Fonts  et  Chaussees,  1859,  vol  i.  p.  355. 
a  Ibid.    1867,  vol.  ii.  pp.  27-115. 

4  Foley's  Du  Travail  en  Vair  comprime,  Paris,  1860. 

5  Proc.  Inst.  C.E.,  vol.  xxi.  p.  265. 

6  Annales  des  Fonts  et  Chaussees,  1864,  vol.  i.  7  See  pp.  159  to  167. 

35 


TUNNEL    SHIELDS 

of  the  bridge  at  Argenteuil  (1859),  however,  Dr.  Foley  published  a  monograph 
giving  his  observations  on  the  effect  of  compressed  air  on  the  health  of  the  men 
engaged  in  sinking  the  caissons  for  the  piers.1  In  it  he  recommends  re-immersion 
in  all  cases  when  a  workman  is  struck  down  after  leaving  the  compressed  air  chamber. 

In  1868,  a  Dr.  Jaminet  was  in  charge  of  the  personnel  employed  in  the  con- 
struction of  the  St.  Louis  Bridge  over  the  Mississippi,  when  the  pressure  in  the 
caissons  reached  50  pounds  to  the  square  inch  on  occasions  :  and  he  subsequently 
embodied  in  book  form  his  observations,  which  from  the  magnitude  of  the  work 
were  made  on  a  scale  considerably  greater  than  those  of  any  previous  investigator.2 

He  recommended  a  sliding  scale  of  working  hours  in  inverse  proportion  to  the 
pressure  of  the  air  as  under  : — 


Pressure  Ibs. 

Hours  of 

Number  of 

Hours  of 

Hours  of 

per  inch. 

Shift. 

Shifts. 

Rest. 

Work. 

15  to  20 

2 

Thrice  a  day 

2  between 

2 

20         25 

2 

»            ,, 

q 

2 

25         30 

2 

Twice  a  day 

3 

2 

30         35 

2 

»            » 

4 

2 

35         40 

1 

Thrice       „ 

2 

1 

40         45 

1 

»            j> 

4 

1 

45         50 

1 

Twice        ,, 

6 

1 

50         55 

1 

Once          ,, 

He  also  recommended  that  the  time  for  entering  the  chamber  through  the  air- 
lock, or  "  locking  in  "  should  be  one  minute  for  every  3  pounds  of  pressure  ;  and 
for  "  locking  out  "  one  minute  for  every  6  pounds  of  pressure.3 

Among  the  600  men  employed  in  the  caissons  of  this  bridge,  there  were  119 
cases  of  compressed  air  sickness,  14  of  which  terminated  fatally  and  two  of  which 
resulted  in  permanent  disablement. 

This  gives  a  very  high  proportion  of  illness  to  the  number  of  men  employed, 
but  the  air  pressure,  50  pounds,  was  of  course  very  high,  and  the  importance  of 
an  abundant  supply  of  air  per  man  per  hour  without  reference  to  the  actual  quantity 
required  for  the  work  was  not  so  fully  recognized  then  as  now. 

In  1871-2,  Dr.  Smith,  whose  work,  Compressed  Air,  summarizes  from  the 
medical  point  of  view  the  main  features  of  caisson  disease  and  its  cure,  was  medical 
officer  to  the  Brooklyn  Bridge,  the  immense  towers  of  which  are  built  in  caissons 
constructed  by  means  of  compressed  air  ;  and  he  appears  to  have  been  the  first  to 
attempt  to  enforce  regulations  devised  to  exclude  unfit  workmen  from,  and  regulate 
the  actions  of  the  men  admitted  to,  the  caissons.  These  regulations  were  similar 
to  those  enforced  in  tunnel  work  at  present,  except  that  the  supply  of  air  provided 
for  appears  to  have  been  inadequate. 

The  maximum  pressure  employed  was  36  pounds,  and  owing  to  the  employ- 
ment of  gas  as  an  illuminant  it  was  found  that  with  a  supply  of  150,000  cubic  feet 
of  free  air  per  hour  with  125  as  a  maximum  number  of  men,  the  amount  of  carbonic 
acid  in  the  air  was  0'3  per  cent.,  or  three  times  as  much  as  more  recent  practice 
considers  satisfactory. 

1  Smith's  Compressed  Air,  pp.  13-15  and  74. 

2  Jaminet's  Physical  Effects  of  Compressed  Air,  St.  Louis,  U.S.A.,  1851. 

3  Smith  recommends  one  minute  for  every  3  pounds  for  '-'  locking  in  "  :    one  minute  for 
every  6    pounds  for  "  locking  out." 

36 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

Under  these  circumstances  110  cases  of  sickness  in  four  months,  three  of  which 
were  fatal,  are  not  more  numerous  than  might  have  been  expected,  the  daily  work- 
ing gangs  numbering  125  men. 

At  the  maximum  pressure,  the  hours  of  work  were  two-hour  shifts  with  four- 
hour  intervals  or  eight  hours  in  a  twenty-four-hour  day. 

To  mitigate  as  much  as  possible  the  effect  of  the  sudden  drop  of  temperature 
on  the  men  when  "  locking  out  "  pipes  heated  by  steam  were  fitted  to  one  of  the 
airlocks  on  this  work.  This  has  not  been  done  again  so  far  as  the  author  is  aware. 

The  most  interesting  portion  of  Dr.  Smith's  book  is  that  in  which  he  suggests 
as  a  practical  remedy  for  cases  of  caisson  sickness,  their  treatment  in  a  specially 
prepared  hospital  or  chamber,  the  air  in  which  could  be  compressed  up  to  the  pres- 
sure of  the  working  chamber  from  which  the  sufferer  had  come. 

He  anticipates,  indeed,  the  arrangements  devised  and  constructed  by  Mr.  E. 
W.  Moir  later  at  the  Hudson  River  Tunnel,1  which  proved  so  successful  there,  and 
elsewhere  since,  in  alleviating  the  suffering  caused  by  caisson  disease. 

"  My  plan,"  2  he  writes  "would  be  as  follows  : — Let  there  be  constructed  of  iron  of  suffi- 
cient thickness  a  tube  9  feet  long  and  3£  feet  in  diameter,  having  one  end  permanently  closed, 
and  the  other  provided  with  a  door  opening  inward,  and  closing  airtight.  This  tube  to  be 
placed  horizontally  and  provided  with  ways  upon  which  a  bed  could  be  slid  into  it.  Very 
strong  plates  of  glass  set  in  the  door  and  in  the  opposite  end  would  admit  the  light  of  candles 
or  gas  jets  placed  immediately  outside.  This  apparatus  should  be  connected  by  a  suitable 
tube  with  the  pipe  which  conveys  the  air  from  the  condensers  to  the  caisson.  An  escape  cock 
properly  regulated  would  allow  the  constant  escape  of  sufficient  air  to  preserve  the  necessary 
purity  of  the  atmosphere  within. 

"  The  bed  containing  the  patient  having  been  slid  into  the  chamber,  the  door  is  to  be 
closed,  and  the  pressure  admitted  gradually  until  it  nearly  or  quite  equals  that  in  the  caisson. 

"  This  should  be  continued  until  the  patient  indicates  by  a  signal  previously  concerted  that 
the  pain  is  relieved.  The  pressure  should  then  be  reduced  by  degrees,  carefully  adjusted  to 
the  effect  produced,  until  at  last  the  normal  standard  is  reached.  By  occupying  several  hours, 
if  necessary,  in  the  reduction  of  the  pressure,  it  is  probable  that  a  return  of  the  pain  could  be 
avoided.  ...  I  should  expect  the  very  best  results  from  it  in  cases  of  extreme  pain  or  in  the 
very  outset  of  paralysis  not  dependent  upon  extravasation  of  blood. 

"  Of  course  the  secondary  conditions  which  arise  in  protracted  cases  would  not  be  capable 
of  direct  relief  by  simply  reproducing  the  physiological  conditions  existing  in  the  caisson. 
The  most  that  might  be  hoped  for  in  such  cases  would  be  that  the  pressure  might  result  in 
giving  a  new  impulse  to  the  circulation  in  the  congested  part,  and  thus  favour  resolution." 

The  arrangement  described  above  is  practically  (except  that  at  the  Hudson 
tunnel  the  air  bath  or  hospital  was  entered  by  means  of  a  lock)  that  first  employed 
by  Mr.  E.  W.  Moir  in  New  York  in  1889.  It  is  to  this  engineer  that  the  credit  of 
bringing  into  general  use  this  most  useful  appliance  belongs. 

The  peculiar  malady  which  is  produced  in  some  conditions  as  a  consequence 
of  prolonged  immersion  in  compressed  air,  is  of  great  interest  from  the  medical 
point  of  view,  but  its  pathological  phenomena  hardly  comes  within  the  scope  of 
this  work.  The  symptoms  may  be  briefly  described,  however,  before  detailing  the 
practical  precautions  against  and  remedies  for  the  disease. 

Caisson  disease  must  not  be  confounded  with  the  purely  mechanical  troubles 
experienced  by  persons  trying  to  go  into  compressed  air,  having  their  eustachian 
tubes,  which  connect  the  cavity  of  the  middle  ear  with  the  external  air,  in  a  blocked 
condition,  caused,  for  example,  by  a  cold  in  the  head.  In  such  a  condition,  the 
increasing  air  pressure  of  the  lock  exerts  its  full  effect  on  the  outside  of  the  ear 

1  Journal  of  the  Society  of  Arts,  May  15,  1896. 

2  Smith's  Compressed  Air,  pp.  74,  75. 

37 


TUNNEL    SHIELDS 

drum,  while  the  pressure  in  the  inside  remains  normal  and  as  a  result  intense  pain 
is  caused  in  the  ears,  which  can  only  be  relieved  by  closing  the  mouth  and  nostrils, 
and  blowing  so  as  to  dislodge  the  obstruction  in  the  tubes  and  so  increase  the  .pres- 
sure inside  the  drum  of  the  ear,  when  the  pain  is  immediately  relieved. 

This  trouble  disappears  altogether  after  a  short  period  when  men  are  working 
regularly  in  compressed  air,  and  there  is  no  danger  of  a  rupture  of  the  drum  of 
the  ear,  even  in  the  case  of  a  man  with  obstructed  tubes  going  into  an  airlock  for 
the  first  time,  provided  that  the  increased  pressure  is  admitted  with  reasonable 
slowness  so  as  to  give  the  sufferer  time  to  make  his  trouble  known. 

The  more  serious  illness  which  is  produced  by  compressed  air  work  carried  on 
under  certain  conditions,  is  known  as  caisson  disease,  and  has  of  late  years  been 
the  subject  of  careful  study  by  competent  observers  (see  footnote  on  page  22). 

Though  the  pathology  of  the  disease  can  hardly  be  said  to  be  perfectly  under- 
stood, the  observations  made  of  it  have  enabled  engineers  and  contractors  engaged 
in  this  class  of  work  to  draw  up  certain  general  regulations  for  the  control  of  their 
men,  and  the  supervision  of  their  health,  which  have  given  beneficial  results. 

Caisson  disease  is  one  "  depending  upon  increased  atmospheric  pressure,  but 
always  developed  after  the  pressure  is  removed.  It  is  characterized  by  extreme 
pain  in  one  or  more  of  the  extremities,  and  sometimes  in  the  trunk,  and  which 
may  or  may  not  be  associated  with  epigastric  pain  and  vomiting.  In  some  cases 
the  pain  is  accompanied  by  paralysis  more  or  less  complete,  which  may  be  general 
or  local,  but  is  most  frequently  confined  to  the  lower  half  of  the  body.  Cerebral 
symptoms,  such  as  headache  or  vertigo,  are  sometimes  present.  The  above  symp- 
toms are  connected,  at  least  in  the  fatal  cases,  with  congestion  of  the  brain  and 
spinal  cord,  often  resulting  in  serous  or  sanguineous  effusion,  and  with  congestion 
of  most  of  the  abdominal  viscera."  1 

The  peculiar  feature  of  the  malady  is  that,  although  caused  by  the  effect  of 
air  pressure  above  the  normal,  it  is  not  until  the  pressure  is  removed  that  the 
symptoms  manifest  themselves,  and,  as  mentioned  above,  in  most  cases  re-immersion 
in  compressed  air  relieves,  at  least  for  a  time,  the  sufferer. 

The  most  common  form  of  the  disease  is  that  known  among  the  miners  as 
"  bends,"  in  which  the  sufferer  is  suddenly  seized  with  pains,  usually  in  the  knees, 
and  of  such  excruciating  character  that  the  strongest  men  are  subdued  by  them. 

In  these  cases  re-immersion  always  gives  relief,  at  any  rate  temporarily. 

In  its  more  serious  form,  the  compressed  air  sickness  ends  in  paralysis,  and 
sometimes  in  death  ;  but  with  increased  knowledge  of  the  causes  of  the  disease 
have  come  improved  methods  of  precaution  against  it,  and  the  more  recent  tunnel- 
ling undertakings  have  been  comparatively  immune  from  serious  cases.  A  careful 
medical  examination  of  all  men  offering  themselves  for  work  in  compressed  air, 
with  rigid  rejection  of  the  unfit,  and  a  re-examination  of  those  accepted  at  least 
weekly,  as  well  as  the  exclusion  of  all  suffering  from  any  temporary  ailment,  have  the 
effect  of  weeding  out  all  men  with  any  natural  defects  for  working  in  compressed 
air  ;  while  an  increased  knowledge  of  the  causes  of  the  disease  has  enabled 
engineers  to  draw  up  regulations  as  to  the  hours  and  conditions  of  work,  and 
the  supply  of  fresh  air,  which  have  greatly  reduced  the  percentage  of  cases  among 
the  men  employed. 

The  principal  causes  of  compressed  air  sickness  are  (1)  excessive  pressure,  (2) 
impurity  of  the  air  in  the  pressure  chamber  and  (3)  too, prolonged  immersion.  It 

1  Smith's  Compressed  Air,  p.  47. 
38 


THE    USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

is  found  that  the  effect  caused  by  any  of  these  three  conditions  is  intensified  by 
the  coincidence  of  the  others. 

To  maintain  the  miners  in  good  health,  an  increase  of  pressure  demands 
greater  purity  in  the  air,  and  shorter  hours  of  labour  ;  bad  air,  that  is  air  having 
an  amount  of  carbonic  acid  of  over  O'l  per  cent.,  is  more  dangerous  in  conjunction 
with  increased  pressure  than  with  a  lower,  and  makes  short  shifts  imperative  :  1 
and  it  is  found  that  the  period  of  safe  working  for  men  varies  above  a  certain  density 
inversely  as  the  pressure,  and  directly  as  the  purity  of  the  air. 

Under  satisfactory  conditions  men  can  work  eight-hour  shifts  with  sixteen 
hours'  rest  per  day  in  pressures  up  to  35  pounds  per  square  inch  ;  an  interval  of  forty 
minutes  for  a  meal  being  given  in  the  middle  of  the  shift.  Above  that  pressure 
the  length  of  the  shift  must  be  reduced  as  the  pressure  increases,  until  at  50  pounds 
pressure,  as  at  the  St.  Louis  Bridge,  the  length  of  the  shift  is  not  more  than  one 
hour. 

The  selection  of  the  men  in  the  first  place,  and  their  re-examination  at  intervals 
afterwards,  should  be  of  course  the  work  of  a  properly  qualified  medical  officer,  who 
after  eliminating  the  obviously  unsuitable  men,  such  as  those  with  weak  hearts,  or 
diseased  lungs,  of  gross  habit  of  body,  or  suffering  from  alcoholism,  should  at  least 
once  a  week  examine  the  working  gangs,  and  weed  out  those  who  show  any  signs 
of  ill  health  or  loose  living. 

At  the  Greenwich  Tunnel  (1899)  it  was  found  necessary  to  reject  13'9  per  cent, 
of  those  presenting  themselves  for  employment  in  the  tunnel,  and  subsequently 
of  those  accepted  in  the  first  place  5-7  per  cent,  were  rejected  after  a  longer  or 
shorter  period  of  work.  The  total  percentage  of  rejections  was  18' 8  of  the  original 
number  examined,  or  very  nearly  one  in  five.  This  proportion  appears  high,  but 
it  is  probable  that  the  knowledge  that  they  would  be  vigorously  examined 
deterred  many  from  presenting  themselves,  and  that  therefore  the  actual  per- 
centage of  working  men  who  are  unfit  to  work  in  compressed  air  is  larger  than  that 
given  by  the  medical  officer's  returns  quoted.2 

When  once  the  men  are  passed  by  the  medical  officer,  the  enforcement  of  any 
regulations  he  may  make  and  of  such  general  rules  as  past  experience  has  shown 
to  be  useful,  are  matters  for  the  engineer  in  charge  of  the  work. 

It  is  usual  to  provide  for  the  men  rooms,  which  can  be  warmed  in  winter,  in 
which  they  can  change  their  clothes  and  rest  after  leaving  the  pressure  chamber  ; 
and  provided  also  with  hot  water,  etc.  A  medical  lock  is  a  necessity,  and  where 
the  number  of  men  employed  is  at  all  large  it  should  be  compulsory  ;  its  cost  is 
a  very  small  item  in  the  equipment  of  the  contractors'  yard. 

When,  as  is  usually  the  case,  the  men  in  coming  out  of  the  pressure  chamber 
have  to  ascend  some  height  to  leave  the  works,  a  lift  should  always  be  provided 
for  them. 

The  medical  officer  in  charge  should  after  each  examination  of  the  men  give 
to  each  ganger  a  signed  list  of  the  men  passed  for  his  gang,  the  ganger  being  then 
made  responsible  for  seeing  that  no  workmen  except  such  as  are  in  the  list  enter 
the  compressed  air  with  his  shift. 

1  Dr.  Hunter,  in  a  thesis  on  Compressed  Air,  presented  for  his  degree  of  M.D.,  now  in  the 
Library  of  the  University  of  Edinboro',  says  in  reference  to  the  sinking  of  the  caissons  at  the 
Forth  Bridge  that  the  worst  conditions  for  the  men  were  ( 1 )  when  they  were  removing  soft  silt, 
containing  much  moisture  and  decaying  matter,  and  (2)  when  concreting  was  going  on,  con- 
siderable generation  of  CO2  taking  place. 

2  Macrnorron,  Notes  in  Caisson  Disease  (privately  printed),  London,  1901. 

39 


TUNNEL    SHIELDS 

The  following  general  rules  for  the  men's  health  should  always  be  en- 
forced :— 

(1)  No  one  suffering  from  any  temporary  illness  from  any  cause  whatever 
which  may  affect  his  normal  state  of  health  should  be  allowed  to  go  into  the  air 
chamber.    This  particularly  applies  to  a  man  under  the  effect  of  excessive  drinking, 
but  any  stomachic  derangement  unfits  a  man  for  compressed  air  work.    Any  case 
of  illness,  even  if  occurring  after  the  man  has  left  the  works,  should  be  reported 
at  once  to  the  medical  officer. 

(2)  No  one  should  enter  the  air  chamber  with  an  empty  stomach.   For  men 
working  in  compressed  air  a  generous  diet  is  necessary. 

(3)  Every  one,  on  leaving  the  pressure  chamber,  and  before  locking  out,  should 
put  on  a  warm  overcoat,  and  on  getting  out  be  supplied  immediately  with  a  hot 
drink,  preferably  of  coffee,  which  is  a  mild  stimulant. 

(4)  Arrangements  should  be  made  so  that  men  coming  out  of  compressed  air 
are  not  required  to  climb  ladders  or  stairways  to  gain  the  surface.     All  exercise  for 
some  time  after  leaving  the  air  chamber  is  unadvisable. 

(5)  When,  as  is  usually  the  case,  an  interval  for  a  meal  of  from  half  an  hour  to 
an  hour  is  given  in  the  middle  of  an  eight-hour  shift,  it  should  be  made  an  in- 
variable rule  that  the  men  should  leave  the  pressure  chamber  for  that  period. 

(6)  The  rate  at  which  the  pressure  is  increased  when  the  men  are  passing 
through   the  lock  into  the  pressure  chamber,  or  "  locking  in  "  as  it  is  termed,  is 
regulated  solely  by  the  men's  convenience ;  the  rate  of  reduction  of  pressure,  how- 
ever, in  "  locking  out  "  is  a  matter  which  should  be  strictly  regulated   by  the 
engineer. 

(Dr.  Smith  goes  so  far  as  to  say  that  if  sufficient  time  were  allowed  for  passing 
out  through  the  lock,  the  disease  would  never  occur,1  and  he  goes  on  to  say  that  at 
least  five  minutes  should  be  allowed  for  each  atmosphere  of  pressure,  or  say  one 
minute  for  every  3  pounds. 

This  appears  a  somewhat  long  "  locking  out."  In  practice,  in  an  ordinary 
lock  about  14  feet  long,  and  6  feet  diameter,  a  pipe  1 J  inch  diameter  is  a  safe  aper- 
ture for  allowing  the  air  to  escape  ;  but  to  ensure  this  being  used,  it  is  necessary  to 
make  stringent  regulations  against  the  employment  of  the  "  muck  tap,"  which  is 
usually  of  3  inches  aperture,  in  passing  men  through  the  lock.) 

(7)  The  amount  of  free  air  supplied  to  the  pressure  chamber  should  be  at  least 
4,000  cubic  feet  per  man  per  hour,  and 

(8)  The  amount  of  carbonic  acid  in  the  air  in  the  pressure  chamber  should 
never  exceed  1  part  in  1,000. 

Nos.  1,  2,  5  and  6  of  the  foregoing  are  matters  for  the  foremen  and  gangers  to 
see  to  ;  Nos.  3,  4,  7  and  8,  and  also  the  provision  of  the  medical  lock,  should  be  the 
subject  of  special  stipulations  in  the  contract  for  the  work,  as  they  involve  an  extra 
outlay  on  the  part  of  the  contractor. 

Properly  warmed  rooms  for  the  men  to  change,  warm  coats  for  locking  out  and 
hoisting  gear  to  bring  the  men  up  the  shafts  are  all  indispensable,  and  should  be 
specified  for. 

The  abundance  and  purity  of  the  air  supplied  to  the  pressure  chamber 
are  so  important  to  the  wellbeing  of  the  men  that  the  minimum  quantity  of 
air  supplied  per  man  per  hour,  which  of  course  regulates  the  purity  also  so  far 

1  Smith's  Compressed  Air,  p.   64. 
40 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

as  relates  to  the  proportion  of  carbonic  acid  contained  in  it,  should  certainly  be 
specified. 

This  is  particularly  necessary  in  cases,  when,  as  in  tunnelling  in  good  material 
but  under  heavy  buildings,  compressed  air  is  used,  not  to  expel  water,  but  merely 
to  hold  up  the  excavation.  In  such  cases,  unless  a  clause  specifying  the  amount  of 
air  to  be  supplied  is  inserted  in  the  contract,  the  contractor  will  not  have  to  pump 
more  air  than  is  sufficient  to  just  maintain  the  pressure  required,  an  amount  which 
may  well  be  below  the  minimum  necessary  for  the  health  of  the  miners. 

When  the  escape  of  air  through  the  surrounding  material  is  less  than  the  amount 
of  fresh  air  necessary  to  keep  the  atmosphere  of  reasonable  purity,  the  only  means 
of  equalizing  the  ingress  and  egress  of  air,  so  as  to  keep  down  the  percentage  of 
carbonic  acid  is  the  waste  or  "  blow-out  "  pipe  which  is  provided  in  all  tunnels  for 
the  purpose  of  blowing  out  by  means  of  the  pressure  in  the  air  chamber  water 
which  may  find  its  way  into  the  tunnel. 

But  this  pipe  is  necessarily  fitted  with  a  valve,  and  consequently  its  use 
depends  on  the  miners  themselves,  who  control  it ;  and  unfortunately,  one  of  the 
main  difficulties  which  those  in  charge  of  tunnelling  in  compressed  air  have  to  over- 
come, is  the  difficulty  of  making  miners  carry  out  orders,  the  object  of  which  they 
imperfectly  understand.  But  it  is  all  important  for  the  health  of  the  men  that  a 
certain  amount  of  free  air  per  man  per  hour  should  pass  into  the  tunnel ;  and 
this  condition  only  becomes  the  more  necessary  as  the  pressure  increases. 

An  increase  of  pressure  accompanied  by  an  increase  of  impurity,  that  is  of 
carbonic  acid,  is  almost  sure  to  prejudicially  affect  the  workmen. 

The  amount  of  carbonic  acid  (C02)  in  the  air  of  the  pressure  chamber  should 
never  exceed  O'l  per  cent.,  or  1  part  in  1,000  ;  if  it  passes  this  limit  an  increase  of 
caisson  sickness  is  to  be  expected. 

The  ordinary  proportion  of  carbonic  acid  in  the  atmosphere  is  0'04  to  0*05  per 
cent. ;  in  an  office  or  room  with  many  occupants  O'lO  per  cent,  or  more  ;  in  mines,  it 
is  said,  it  is  sometimes  as  high  as  0'75  per  cent. 

At  the  Greenwich  Tunnel  (1899)  careful  observations  were  made  by  the  chemi- 
cal staff  of  the  London  County  Council,  and  it  was  found  that  on  the  average  of 
twenty-four  analyses  of  the  air  made  regularly  during  three  months,  with 
an  air  pressure  of  22  J  pounds  and  a  temperature  averaging  6|  degrees  above 
the  outside  air,  the  percentage  of  carbonic  acid  in  the  pressure  chamber  was  0'0786 
per  cent,  as  against  0'0475  in  the  engine  room  where  the  air  was  taken  in  by  the 
compressors.  To  maintain  this  degree  of  purity,  an  average  of  5,774  cubic  feet  of 
free  air  per  man  per  hour  was  pumped  into  the  tunnel. 

The  amount  pumped  never  fell  below  4,100  cubic  feet  per  man  per  hour  ;  and 
all  the  rules  suggested  above  as  necessary  were  put  in  force. 

Under  these  circumstances  the  health  of  the  men  was  very  satisfactory,  only 
nine  cases,  three  of  them  serious  but  none  fatal,  of  caisson  sickness  occurring 
during  the  thirteen  months  during  which  work  was  carried  on  under  compressed 
air. 

At  the  Blackwall  Tunnel  (1892-7),  where  careful  observation  was  also  kept  on 
the  men  and  the  conditions  of  work  in  compressed  air  were  the  same,  a  proportion- 
ately equal  immunity  from  serious  illness  resulted.  Dr.  Snell,  the  Resident  Medi- 
cal Officer  of  the  London  County  Council  on  the  work,  whose  book,  Compressed 
Air  Illness,  before  referred  to,  details  his  observations  at  this  tunnel,  states  that 
during  the  period  of  construction  only  two  hundred  cases,  none  of  them  fatal  and 

41 


TUNNEL    SHIELDS 

many  of  them  trivial,  of  caisson  disease,  occurred.  This  tunnel  was  more  than 
twice  the  diameter  of  the  Greenwich  one,  and  probably  the  working  gangs 
employed  in  it  were  at  any  time  five  times  as  numerous  as  those  at  Greenwich. 

The  results  obtained  in  these  two  cases  are  the  most  satisfactory  yet  recorded 
in  compressed  air  work. 

It  is  interesting  to  note  that  at  both  tunnels  cases  of  caisson  sickness  occurred 
owing  to  excess  of  carbonic  acid  at  periods  when  the  air  pressure  in  the  tunnel 
was  considerably  below  the  maximum. 

At  Greenwich,  where  the  maximum  pressure  was  about  28  pounds,  three  out 
of  the  nine  cases  recorded  occurred  at  a  time  when  the  pressure  was  only  12  pounds, 
and  at  Blackwall,  when  the  maximum  pressure  reached  37  pounds,  cases  of  sickness 
occurred  at  a  lower  pressure  and  with  an  air  supply  of  6,500  feet  per  man  per  hour. 
In  both  cases  there  were  causes  extraneous  to  the  amount  of  pressure  which  caused 
the  increase  in  sickness.  At  Greenwich  the  men  affected  were  all  suffering  from 
severe  colds  ;  and  at  Blackwall  they  were  working  in  a  compartment  of  the  shield 
where  an  explosive  generating  carbonic  acid  was  in  use.  These  cases  confirm 
the  results  of  earlier  experience  stated  above,  namely,  that  impurity  of  the  air 
and  bad  conditions  of  health  of  the  men  count  almost  as  much  as  the  density 
of  the  air  in  producing  caisson  sickness. 

At  Blackwall  Tunnel,  Dr.  Snell  made  careful  observations  with  the  view  of 
determining,  if  possible,  the  general  conditions  to  be  laid  down  for  the  safe  pro- 
secution of  similar  work  in  the  future. 

His  observations  as  to  the  effect  of  increased  pressure  on  the  health  of  the 
men  merely  laid  down  in  exact  form  conclusions  generally  accepted,  but  they  are 
interesting  as  the  first  series  of  tabulated  observations  on  a  sufficiently  extended 
scale  to  give  general  value  to  his  results. 

His  table  *  giving  the  percentages  of  cases  of  sickness  occurring  on  215  days  on 
which  the  air  pressure  was  but  little  above  or  little  below  20  pounds,  and  the  length 
of  the  shifts  eight  hours  each  shows  clearly  the  effect  of  increased  air  supply  in 
the  men's  health. 


Free  Air  pumped  per  Man  per  Hour 
Cubic  Feet. 

No.  of 
Days. 

No.  of 
Cases. 

No.  of  Cases 
per  100  Days. 

Below  4,000      

56 

16 

28-5 

4,000  to  8,000        

47 

9 

19-1 

8,000  to   12,000      
Above  12,000 

71 
41 

8 
0 

11-2 

0- 

In  this  table  (one  of  several)  of  the  three  conditions  affecting  the  sickness, 
amount  of  pressure,  length  of  immersion,  and  amount  of  air,  two  are  constant, 
and  the  third,  the  amount  of  air  pumped  per  man  per  hour,  is  a  variant,  with  the 
result  that  the  amount  of  sickness  is  seen  to  vary  inversely  with  it. 

Dr.  Snell  also  observed,  so  far  as  he  was  able,  the  proportion  of  sickness  among 
men  of  different  ages,  and  from  his  figures  it  would  seem  that  while  men  under 
twenty  are  immune  from  the  disease,  men  of  all  ages  from  twenty  to  forty  are  about 
equally  susceptible  to  it,  while  men  over  forty-five  are  entirely  unsatisfactory. 


1  Snell's  Compressed  Air  Illness,  p.   141. 
42 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

His  figures  are  as  under  : — l 


Men's  Age. 

No.  of  Men  Passed. 

Cases  of  Sickness. 

Cases  per  cent. 

15  to  20 

55 

0 

0 

20         25 

145 

15 

10-3 

25         30 

152 

37 

24-3 

30         35 

91 

19 

20-9 

35         40 

61 

14 

22-9 

40         45 

38 

10 

26-3 

45         50 

3 

5 

166 

The  figures  are  doubtless  correct,  but,  even  if  the  men  over  forty-five  years  of 
age  be  ignored  in  making  the  calculation,  it  would  appear  that  after  the  men  had 
been  passed  by  the  doctor  over  17  per  cent,  were  affected  by  the  compressed  air 
work,  a  high  percentage  considering  that  only  for  a  short  time  in  sinking  the  shafts 
did  the  pressure  reach  35  pounds. 

The  figures  in  this  table  do  not,  however,  give  the  total  number  of  men  passed 
or  injured,  but  only  those  whose  ages  were  known. 

One  reason  of  the  high  percentage  of  the  table  may  be  that  at  the  Blackwall 
Tunnel  it  was  optional  for  men,  once  passed  by  the  doctor  as  fit  to  work  in  com- 
pressed air,  to  present  themselves  for  inspection  again.  The  regular  weekly 
inspections  insisted  on  at  the  Greenwich  Tunnel  a  little  later  resulted  in  the 
weeding  out  of  5' 7  per  cent,  of  those  who  had  previously  got  past  the  doctor. 

The  London  County  Council,  fixed  in  the  cases  of  these  two  tunnels,  as  also 
in  the  one  now  in  course  of  construction  at  Rotherhithe,  the  minimum  amount  of 
free  air  to  be  pumped  per  man  per  hour  at  8,000  cubic  feet.  Much  less  than 
this,  however,  if  actually  pumped  (for  specifications  and  contractors'  actual 
compliance  with  them  are  somewhat  different),  should  keep  the  air  purer  than  that 
of  most  workshops. 

The  general  conditions  of  compressed  air  work  laid  down  in  the  specification 
for  the  Rotherhithe  Tunnel  now  commencing  are  so  complete  as  regards  the  secur- 
ing of  healthy  conditions  for  the  men,  and  also  cover  the  ground  so  well  in  the  other 
matters  connected  with  the  compressed  air  plant  of  the  tunnel  that  they  are  given 
in  extenso  below.2 


EXTRACT  FROM  THE  SPECIFICATION  ATTACHED  TO  THE  CONTRACT  FOR  THE 

ROTHERHITHE  TUNNEL. 

Clause  25.     Throughout  the  whole  of  the  time  occupied  by  these  works  the  Plant. 

Contractor  shall,  without  extra  charge,  provide  in  duplicate  for  each  working 
face  sufficient  hydraulic  machinery,  airpumps,  engines,  airlocks,  grouting 
apparatus,  etc.,  and  he  shall  keep  the  same  ready  for  use.  He  shall  make  sure 
that  in  case  of  a  breakdown  in  one  set  of  appliances  another  set  can  immediately 
be  used  in  its  place,  each  set  being  by  itself  fully  capable  of  doing  the  maximum 
work  which  has  to  be  done  in  the  most  extreme  cases. 

26.  The  Contractor  shall,  without  extra  charge,  have  all  the  working  faces 
properly  ventilated,  and  the  amount  of  carbonic  acid  gas  present  at  any  time 
shall  not  be  allowed  to  exceed  0'08  per  cent.  A  minimum  of  8,000  cubic  feet 
of  free  air  per  hour  per  man  shall  be  pumped  into  the  tunnel,  and  shall  be  brought 
to  the  working  face.  When  working  in  compressed  air  at  the  bottom  of  the 
caisson,  similar  conditions  shall  apply.  The  blow-out  pipe  shall  be  used  at  the 

1  Snell's  Compressed  Air  Illness,  p.   154. 

2  Compare  the  specification  for  compressed  air  work  in  the  East  Boston  Tunnel,  chap.  ix. 

43 


Ventilation 

of  working 

faces. 


TUNNEL  .  SHIELDS 

working  faces  once  at  least  in  every  hour.     Suitable  lifts  and  resting  places  for 

the  men,  including  a  compressed  air  chamber  fitted  with  bunks,  and  a  drying 

room  for   clothes   must   be   provided.     In  all  airtight  floors  and  bulkheads  for 

Emergency         the  shafts  and  tunnel,  a  small  emergency  airlock  must  be  provided,  in  addition 

exits.  ^0  the  ordinary  working  airlock,  with  access  thereto  from  the  ordinary  working 

levels. 

27.  The  tunnel  and  shafts  shall,  without  extra  charge,  be  lighted  by  electric 
light  during  the  progress  of  the  works  ....  and  generally  the  Contractor  shall 
provide  every  means  and  appliance  which  may  in  any  way  conduce  to  the  safety 
of  the  works  and  the  men  employed  in  them. 

Refreshments  28.     Whenever  possible  each  man  coming  out  of  the  compressed  air  chamber 

mentsTfor6"        shall  be  provided  with  a  cup  of  hot  coffee,  and  arrangements  shall  be  made  that 
men.  it  shall  not  be  necessary  to  climb  any  stairs  immediately  after  coming  out.     Pro- 

per sanitary  conveniences,  in  all  respects  satisfactory  to  the  engineer,  shall  be 
made  for  the  men  working  in  the  tunnel  and  shafts.  The  greatest  care  shall  be 
taken  that  all  portions  of  the  work  being  carried  out,  whether  under  compressed 
air  or  otherwise,  shall  be  kept  in  a  thoroughly  sanitary  condition.  The  carrying 
condition  ou^  °^  ^e  wh°le  °f  the  conditions  of  this  clause  shall  be  considered  as  a  contin- 

gency on  the  cost  of  the  work. 

~\/f    *-1  *        1 

Offi  C<  ^'     ^e  Council  may  engage  the  services  of  a  qualified  medical  practitioner 

to  look  after  the  well-being  of  the  men  employed,  and  should  they  do  so  the 
Contractor  shall,  without  extra  charge,  follow  out  all  the  reasonable  instructions 
of  the  same  from  time  to  time. 

Fitness  of  30.     No  workman  shall  be  engaged  for  the  compressed  air  work  without  his 

fitness  for  such  duties  being  proved  by  such  medical  examination  as  the  Council 
may  direct. 

In  addition  to  providing  in  the  specification  for  the  health  of  the  men  working 
in  compressed  air,  special  clauses  were  inserted  in  the  Act  authorizing  the  con- 
struction of  the  tunnel,  whereby  the  Council  was  authorized  to  pay,  in  the  cases 
of  men  working  in  compressed  air,  compensation  for  injury  caused  by  such  work, 
and  these  are  given  below  : — 

(63  and  64  Viet.)  THAMES  TUNNEL.     (Ch.  CCXIX.) 
(ROTHERHITHE  AND  RATCLIFFE) 

ACT  1900 

Compensation  55.     The  Council  shall  have  power  in  their  discretion  to  pay  compensation 

to  workmen        to  any  workman  or  person  employed  in  the  construction  of  the  tunnel  who  may 

m  special          be  injured  by  reason  of  working  under  compressed  air,  and  to  the  widow  and 

children  or  any  of  them  of  any  such  workman  or  person  who  while  so  working 

as  aforesaid  shall  die  or  sustain  injury  resulting  in  death. 

Such  compensation  as  aforesaid  may  be  paid  either  in  one  sum  or  by  period- 
ical payments  at  such  times  and  extending  over  such  period  as  the  Council  may 
think  fit,  and  the  Council  may  if  they  think  fit  contract  (for  such  consideration 
to  be  paid  by  the  Council  as  they  may  think  proper)  with  any  insurance  office 
society  or  company  for  the  payment  by  such  office  society  or  company  of  any 
such  compensation  as  aforesaid. 

The  expenses  of  the  Council  under  this  Section  shall  be  considered  as  ex- 
penses incurred  by  them  in  the  compensation  of  the  tunnel,  and  shall  be  defrayed 
accordingly. 

Nothing  in  this  Act  and  no  compensation  which  may  be  paid  or  become  pay- 
able thereunder  shall  take  away  or  prejudicially  affect  any  right  or  claim  to 
damages  or  compensation  which  any  such  workman  or  person  as  aforesaid  or 
his  widow  or  children  may  have  in  respect  of  any  accident  against  any  person 
or  body. 

Various  methods  have  been  suggested  for  removing  from  the  air  of  the  tunnel 
the  carbonic  acid  which  affects  so  unfavourably  the  health  of  the  workmen 
employed  in  tunnels  constructed  with  the  aid  of  compressed  air,  but  hitherto 
no  attempt  has  been  made  to  purify  the  air  pumped  into  a  tunnel  on  a  scale 
large  enough  to  deduce  any  definite  results. 

44 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 

The  simplest  way  of  taking  up  the  free  carbonic  acid  in  the  air  is  by  passing 
the  air  through  or  over  a  substance  like  lime  or  caustic  soda  for  which  carbonic  acid 
has  a  strong  affinity.  But  there  are  practical  difficulties  in  the  way  of  applying 
this  well  known  chemical  action  to  actual  work  in  the  engine-room.  In  the  first 
place  the  volume  of  air  pumped  in  a  large  tunnel  is  so  great  and  the  section  of  the 
delivery  pipe  so  small  in  proportion,  that  the  flow  of  air  through  any  scrubber  or 
similar  appliance  filled  with  a  saturated  solution  of  lime  and  inserted  between  the 
air  compressors  and  the  tunnel,  is  too  rapid  for  any  effectual  cleaning  to  be  possible 
unless  the  cleaning  apparatus  extended  to  a  length  and  bulk  which  very  few 
contractors'  yards  would  admit  of. 

If  the  purifying  apparatus  were  placed  outside  the  compressors  a  special 
arrangement  of  the  compressors  and  engine-room  would  be  necessary  to  ensure 
that  all  the  air  pumped  had  previously  passed  through  the  purifiers. 

The  placing  of  purifying  tanks  in  the  pressure  chamber  of  the  tunnel  is 
impossible  :  considerations  of  space  forbid  it. 

When  acting  as  Resident  Engineer  of  the  Greenwich  Tunnel,  the  author  was 
requested  by  the  Chief  Engineer  of  the  London  County  Council  to  inquire  into  the 
possibility  of  purifying  the  air  in  the  tunnel  by  eliminating  some  of  the  carbonic 
acid  (which  in  ordinary  air  amounts  to  about  0'05  per  cent,  by  volume)  from  the 
air  pumped  into  the  tunnel ;  and  the  London  County  Council  made  a  grant  of 
money  for  carrying  out  experiments  with  that  object.  At  the  time  the  experi- 
ments were  authorized  the  tunnel  was  more  than  half  completed,  and  no  serious 
alteration  of  the  air-compressing  plant  was  possible.  The  engines  were  built 
by  Messrs.  Walker  Brothers,  and  were  entirely  satisfactory  in  working  ;  but  from 
their  construction  and  the  arrangement  of  the  engine  house,  it  was  not  possible  in 
any  way  to  box  them  in  so  as  to  arrange  that  all  the  air  entering  the  compressing 
cylinders  should  first  pass  through  a  purifying  chamber,  or  an  installation  similar 
to  the  purifiers  or  scrubbers  used  in  gas  making.  It  was  equally  impossible  to 
interpose  a  purifying  chamber  between  the  engines  and  the  tunnel,  even  if  it  had 
been  permissible  to  incur  the  risk  of  a  temporary  break  in  the  supply  of  air.  Con- 
sequently the  experiment  was  limited  to  trying  what  could  be  done  in  the  tunnel 
itself  under  working  pressure. 

Under  these  circumstances,  the  use  of  lime  in  solution  was  prohibited  by  rea- 
sons of  space,  but  a  saturated  solution  of  caustic  soda,  which  bulk  for  bulk  is  more 
effective  in  taking  up  carbonic  acid  than  lime,  was  experimented  with,  and  although 
the  limited  space  available  rendered  it  somewhat  doubtful  whether  any  marked 
results  could  be  obtained  by  its  use,  a  purifier  of  somewhat  primitive  design  was 
constructed,  and  the  results  obtained  with  it  were  observed  by  officials  of  the 
Chemists'  Department  of  the  London  County  Council,  who  had  also  previously 
from  time  to  time  made  analyses  of  the  air  in  the  tunnel. 

The  purifier  consisted  of  two  rectangular  trunks  of  wood,  one  above  the  other, 
open  at  one  end,  and  having  sliding  doors  at  the  other  end.  The  ends  fitted  with 
doors  were  connected  with  the  air  inlet  of  the  tunnel  by  a  conical  box,  the  connexion 
with  the  airpipe  being  made  airtight  by  a  flexible  joint.  By  opening  one  or  other 
of  the  sliding  doors,  the  air  was  made  to  pass  through  either  the  upper  or  the  lower 
trunk,  as  required.  Each  trunk  had  one  side  removable,  and  each  contained  eight 
removable  wire  boxes  containing  pumice  stone  broken  small,  which,  before  being 
put  into  the  trunks,  were  dipped  in  a  saturated  solution  of  caustic  soda.  When 
the  upper  trunk  was  filled  with  freshly  dipped  boxes,  the  door  of  the  lower  trunk 

45 


TUNNEL    SHIELDS 

was  closed,  and  its  boxes  were  taken  out,  dipped  and  replaced.  The  bottom  door 
was  then  opened,  and  the  top  one  closed,  thus  diverting  the  course  of  the  air  from 
the  upper  to  the  lower  trunk  ;  the  boxes  in  the  upper  trunk  could  then  be  removed 
and  dipped  afresh  in  the  caustic-soda  solution.  A  constant  relay  of  fresh  boxes 
was  kept  up  day  and  night  ;  and  the  deposit  of  carbonate  of  soda  on  the  pumice 
stone  soon  showed  that  some  effect  was  produced  upon  the  air. 

The  purifier  was  in  operation  for  a  month,  and  analyses  of  the  air  in  the  shield 
were  made  on  fourteen  different  dates.  The  results  are  given  in  the  Table  at  the 
end  of  this  chapter  which  shows,  in  addition  to  the  amount  of  carbonic  acid  in  the 
air,  the  air  pressure,  the  temperature  of  the  tunnel  as  compared  with  that  of  the 
engine-room,  and  the  amount  of  air  supplied  per  man  per  hour,  on  the  days  when 
the  tests  were  made.  It  will  be  seen  that  after  the  purifier  had  been  taken  into 
use,  the  average  proportion  of  carbonic  acid  present  in  the  tunnel  showed  a 
decrease  on  that  observed  before.  If  there  were  no  other  factors  to  be  con- 
sidered, such  a  result  would  be  conclusive,  but  there  are  so  many  circumstances 
which  modify  the  result  that  the  author  would  confine  himself  to  saying  that  the 
apparatus  employed  was  responsible  for  some  of  the  improvement  ;  and  that,  as 
some  effect  was  produced  by  its  use,  a  similar  purifying  apparatus  properly 
elaborated  might  reasonably  be  expected  to  eliminate  the  excess  portion  of  the 
carbonic  acid  in  the  air. 

The  last  two  lines  of  the  Table  give  the  averages  for  all  the  observations  before 
and  after  fitting  up  the  purifier.  The  average  diminution  of  CO2  by  volume,  appar- 
ently consequent  upon  the  use  of  caustic  soda,  is  O'Ol  per  cent.  The  average  amount 
of  free  air  supplied  per  man  per  hour  was  a  little  less,  and  the  pressure  per  square 
inch  3 1  pounds  less,  after  the  purifier  was  put  in,  than  previously  :  so  that  these 
two  important  factors  in  determining  the  amount  of  C02  in  the  air  do  not  vary 
much.  The  variations  in  the  quantity  of  CO2  in  the  free  air  of  the  engine-room 
were  probably  caused  by  momentary  contamination  of  the  air,  due  to  the  proximity 
of  the  boiler  furnaces.  The  samples  of  air  in  the  tunnel  were  taken  at  floor  level  ; 
and  the  percentages  given  in  the  Table  are  the  averages  of  two,  and  sometimes  of 
three,  separate  samples,  one  of  which  was  always  taken  a  few  yards  behind  the 
shield.  On  the  whole,  the  results  of  the  analyses  are  encouraging  ;  they  appear 
to  show  that  the  purifier  produced  some  beneficial  effect. 

The  experiment,  however,  was  inconclusive,  owing  to  the  conditions  which 
prevailed.  When  the  observations  were  taken  the  pressure  was  comparatively 
low,  and  the  amount  of  air  supplied  to  the  men  considerably  in  excess  of  that 
provided  in  most  compressed  air  undertakings. 

It  would  be  interesting  for  some  further  experiments  to  be  made  with  caustic 
soda  under  more  severe  conditions  of  pressure  and  quantity  of  air,  and  this  could 
be  done  at  comparatively  small  cost. 

The  expense  of  the  experiment  made  by  the  author  was  about  £1  per  day, 
but  with  a  little  more  elaboration  of  the  plant  this  amount  should  be  reduced. 

The  temperature  of  the  air  supplied  to  the  pressure  chamber  is  of  course 
considerably  raised  above  that  of  the  free  air  of  the  engine-room  where  it  is  taken 
into  the  compressors  by  the  very  act  of  compression,  and  the  friction  of  the 
delivery  pipes  through  which  it  rushes  with  considerable  velocity  augments  its 
heat.  This  is  remedied  in  part  by  passing  the  air  through  a  cooling  tank  or  air 
reservoir  the  shell  of  which  is  kept  cool  by  jets  or  streams  of  water  playing  on  it. 

46 


THE  USE  OF  COMPRESSED  AIR  IN  ENGINEERING  WORK 


TABLE  GIVING  THE  RESULTS  OF  SOME  EXPERIMENTS  MADE  AT  GREENWICH  FOOTWAY  TUNNEL 
IN  1901,  WITH  A  VIEW  TO  TESTING  CAUSTIC  SODA  AS  A  PURIFIER. 


Tempera- 

ture  in 

Percentage  of  CO^  by  Volume. 

Air-                 Shield 

Pressure 

above  or         Free  Air 

Date. 

per 

below             Supplied 

Remarks. 

Square             that  of           per  man 

Inch.               Engine-           per  hour.        In  Engine- 

In  Tunnel.         In  Shield. 

luum. 

ruum. 

Fahr. 

, 

1901. 

Lbs. 

Degrees. 

Cub.  Ft. 

March    5 

24 

+  10 

6,000 

0-047 

0-073 

0-196 

Men  working. 

„     11  I         21 

+  19 

5,280 

0-055 

0-097 

0-218 

End  of  shift. 

„     18 

24 

+  15 

5,600 

0-045 

0-064 

0-189 

»                   ,, 

|  After      blow-o  u  t 

»     » 

— 

— 

— 

— 

— 

0-124 

pipe   had   been 

I     used. 

[  End      of     shift; 

„     25 

23 

+  15 

5,280 

0-047 

0-058 

0-125 

I        blow-out  pipe 
1        used    four 

\       times. 

{After      blow-out 

,,     ,, 

—  - 

— 

— 

— 

— 

0-098 

pipe  had  been 

used. 

C  Men       working  ; 

April      1 

25 

+  13 

5,280 

0-051 

0-087 

0-186 

I        blow-out    pipe 
1        used   at  inter- 

l       vals. 

„     15 

•  26 

+   6 

5,280 

0-049 

0-084 

0-173 

» 

„     22 

24 

-   7 

5,280 

i 

0-098 

0-223 

— 

„     29 

221 

-10 

5,280 

0-043 

0-080 

0-170 

f  Blow-out  pipe  in 
[       use. 

May     6 

17 

+    5 

7,000 

0-049 

0-076 

— 

Shield  closed. 

„     13 

18 

—   2 

7,460 

0-042 

0-069 

0-100 

— 

1  i 

/   Purifier  in  work- 

» 

(       ing  order. 

„     20 

19 

—   2 

5,900 

i 

0-056 

0-097 

— 

„     22 

17 

-   3 

6,000 

0-067 

0-074 

0-111 

— 

„     24 

16 

-   3 

6,870 

i 

0-072 

0-126 

— 

„     27 

18£ 

-   7 

7,195 

2 

0-066 

— 

.  — 

„     29 

17 

-   9 

6,580 

0-047 

0-077 

— 

— 

„     31 

23 

+    1 

5,280 

0-052 

0-073 

— 

— 

June       3 

21 

+   5 

5,280 

0-049 

0-075 

—  .  •. 

„       6 

20 

+    1 

5,430 

0-068 

0-077 

— 

7 

18 

+    1 

5,430 

0-059 

0-057 

— 

— 

",    10 

20 

—    5 

4,260 

0-042 

0-066 

— 

„     11 

21 

— 

4,240 

0-039 

0-072 

Doubtful. 

,,     12 

20 

+   5 

4,100 

0-045 

0-068 

—                        — 

„     13 

19£ 

+   6 

4,500 

0-039 

0-062 

— 

— 

„     14 

isf 

+   2 

4-260 

0-049 

0-068 

— 

— 

— 

224 

+   6i 

5,774 

0-0475 

0-0786 

f   Average     before 
\        using  purifier. 

— 

19 

5,330 

0-0505 

0-0688 

— 

(    Average       when 
I        using  purifier. 

Sample  spoilt. 


2  Sample  spoilt.     Shield  in  south  shaft. 


47 


Chapter   III 


CAST-IRON  LINING  FOR  TUNNELS 

ITS  USE  IN  TUNNELS  SUGGESTED  BY  ITS  EMPLOYMENT  IN  PIT  SHAFTS — TELFORD'S  IRON  CENTRES, 
1824 — RHIZA'S  IRON  CENTRES  AND  FACE  JACKS,  1860 — EASE  OF  CONSTRUCTION  AND 
IMMEDIATE  SECURITY  ENSURED  BY  ITS  USE — CIRCULAR  TUNNELS  MOST  CONVENIENT  WHEN 
CAST-IRON  LINING  is  USED — PROPORTIONS  OF  THE  CAST-IRON  SEGMENTS — EXAMPLES 
FROM  RECENT  WORK — THE  KEY — THE  JOINTS — CENTRAL  LONDON  RAILWAY — WATERLOO 
AND  CITY  RAILWAY — BAKER  STREET  AND  WATERLOO  RAILWAY— BLACKWALL  TUNNEL 
— GREENWICH  TUNNEL — ST.  CLAIR  TUNNEL — GREAT  NORTHERN  AND  CITY  RAILWAY 

— ROTHERHITHE     TUNNEL LEA    TUNNEL CASTING     OF    TUNNEL     SEGMENTS  THE 

BRITISH  HYDRAULIC  COMPANY'S  MOULDING  MACHINE — TABLES — QUANTITIES  PER  YARD 
FORWARD  OF  SOME  TYPICAL  IRON-LINED  TUNNELS 

Cast-Iron   Lining  for  Tunnels 

THE  use  of  cast  iron  built  up  in  successive  rings,  each  comprised  of  several 
segments,  as  a  material  for  tunnel  construction  only  dates  from  1869,  when 
it  was  employed  in  the  Tower  Subway,  but  Brunei  had  proposed  its  use  in  1818, 
and  it  is  possible  that  the  Thames  Tunnel  would  have  been  constructed  in  iron, 
had  the  circular  section  first  proposed  been  adopted.  As  is  known,  a  rectangular 
tunnel  was  decided  on,  and  almost  as  a  necessary  consequence  a  masonry  tunnel 
was  preferred  to  a  cast-iron  one. 

For  lining  shafts,  however,  cast  iron  has  been  employed  for  more  than  one 
hundred  years,  and  no  doubt  that  use  of  it  suggested  its  employment  in  tunnels 
to  Brunei. 

In  1795  "  tubbing  in  circles  "  was  used  for  the  first  time  at  the  Walker  Colliery 
on  Tyneside,  and  in  1796,  tubbing  made  of  cast-iron  segments  was  put  in  the  shaft 
of  Percy  Main  Colliery.1  The  use  of  cast-iron  lining  or  tubbing  in  shafts  in  water- 
bearing strata  has  since  been  the  universal  practice  in  the  North  of  England,  the 
portion  of  the  shafts  in  solid  ground  being  brick-lined  in  the  ordinary  way. 

Although  not  used  as  a  permanent  lining  to  tunnels  until  1869,  the  convenient 
way  in  which  frames  of  cast  iron  could  be  built  rapidly  and  securely  for  centering 
and  the  like,  and  be  with  equal  facility  taken  down,  recommended  it  early  in  the 
last  century  to  Telford,  who  in  the  second  Hardcastle  Tunnel,  built  in  1824,  con- 
structed centres  made  up  of  sixteen  segments  bolted  together,  which  could  be  used 
again  and  again  ;  2  and  about  1860,  a  M.  Rhiza  invented  and  used  in  various 
tunnels  in  Central  Europe  a  system  of  centres  and  of  "  face  rams  or  jacks  "  in 
combination  with  them  which,  save  for  lack  of  mobility,  presents  many  of  the 
advantages  of  the  shield  method  ;  3  indeed,  he  worked  out  his  system  with  such 

1  Society  of  Engineers,  1893.     Collieries  and  Colliery  Engineering,  by  R.  Nelson  Boyd. 

2  Rickman's  Life  of  Telford.     See  plates. 

3  Drinker's  Tunnelling,  p.   680,  et  seq. 

48 


CAST-IRON    LINING    FOR    TUNNELS 

detail  and  completeness,  that  it  is  surprising  he  stopped  short  of  the  use  of  iron 
as  a  permanent  lining. 

The  great  advantage  iron  tunnel  lining  has  over  masonry  construction  is  that 
it  attains  its  full  strength  at  once,  and  that  the  process  of  erecting  it  is  simple, 
rapid,  and  easily  supervised  ;  so  that  given  fairly  satisfactory  castings,  it  is  easy  to 
ensure  a  sound  job. 

For  use  in  tunnels  constructed  with  shields  the  first  qualification  is  the  more 
important  ;  the  advance  of  the  shield  being  relatively  so  rapid  that  a  masonry 
lining  has  not  time  to  set  properly  before  it  is  called  on  to  sustain,  not  only  the 
pressure  of  the  ground  around  it,  but  also  the  back  thrust  of  the  shield  rams. 
Various  methods  have  been  tried  to  obviate  this  latter  difficulty,  and  some  will  be 
considered  in  describing  the  use  of  roof  shields  for  large  masonry  tunnels,  but  in 
any  case  the  pressure  of  the  ground  above  must  be  sustained  immediately  the  shield 
is  moved  forward,  and,  if  the  tunnel  to  be  driven  passes  undei  heavy  and  valuable 
buildings  the  extra  cost  of  an  iron  lining  is  well  repaid  by  the  increased  security 
against  damage  obtained  by  its  use. 

The  actual  ratio  of  the  cost  of  iron  lining  to  that  of  masonry  varies  considerably 
with  the  size  of  the  tunnel. 

Speaking  generally,  the  larger  the  tunnel  the  less  the  cost  of  an  iron  lining 
exceeds  that  of  brickwork  ;  not  so  much  on  account  of  the  comparative  approxima- 
tion in  cost  of  the  two  materials  as  the  diameter  of  the  tunnel  increases,  but  on 
account  of  the  saving  in  the  amount  of  excavation  made  by  the  use  of  a  cast-iron 
lining.  In  a  tunnel  for  a  single  line  of  railway,  like  the  Hudson  Tunnel  shown  in 
Fig.  102,  the  proportions  of  the  area  excavated  to  the  area  of  the  inside  of  the 
tunnel  in  the  case  of  an  iron-lined  and  brick-lined  tunnel  respectively,  I" 22  and 
1-60  to  1-00. 

Especially  do  the  foregoing  remarks  apply  to  tunnels  driven  through  water- 
bearing strata,  where  masonry  is  particularly  difficult  to  construct  in  a  sound 
manner,  while  an  iron-lined  tunnel  can  be  made  reasonably  watertight  with  very 
little  trouble,  however  great  the  pressure  due  to  the  head  of  water  outside  may 
be. 

The  universal  practice  in  England  since  1868  has  been  to  use  the  shield  in  con- 
junction with  a  cast-iron  lining  to  the  tunnel,  of  circular  section,  and  composed  of 
successive  rings,  which  again  are  made  up  of  a  number  of  segments,  and  a  closing 
piece  or  key.  (For  a  typical  cast-iron  tunnel,  see  Fig.  30.) 

This  section  is  the  most  suitable  for  iron-lined  tunnels  for  several  reasons. 
In  the  first  place,  the  circular  section  when  the  tunnel  is  made  in  fairly  solid  homo- 
geneous material  is  economically  the  best,  as,  save  for  the  inequality  caused 
by  the  difference  of  level  between  the  crown  and  invert,  the  pressure  on 
the  lining  is  normal  to  the  circle.  In  semi-fluid  material  of  different  densities 
there  is,  of  course,  no  means  of  determining  the  economical  sections  for  every  case. 
In  the  second  case  the  erection  of  the  cast-iron  lining  would  be  made  much  more 
troublesome  and  slower  if  in  order  to  obtain  a  section  other  than  a  circular  one 
cast-iron  segments  of  different  shapes  were  employed.  In  a  circular  tunnel,  with 
the  exception  of  the  two  immediately  adjoining  the  key  (and  even  those  in  some 
cases)  all  the  segments  are  interchangeable,  and  consequently  no  time  is  lost  in 
sorting  out,  in  the  dim  light  in  which  tunnelling  operations  are  necessarily  carried 
on,  the  different  pattern  of  segment  required  for  each  part  of  the  lining.  In  the 
third  place  it  is  easy,  if  the  tunnel  be  circular,  to  "  break  joint  "  with  successive 

49  E 


TUNNEL    SHIELDS 


rings,  and  so  greatly  increase  the  stiffness  of  the  tunnel,  without  increasing  the 
number  of  patterns  required. 

And  finally  a  circular  tunnel  is  convenient  in  view  of  the  fact  that  every  shield 
rotates  more  or  less  on  its  horizontal  axis,  and  that  consequently  a  circular  section 
is  the  only  one  which  will  be  always  the  same,  relatively  to  a  vertical  axis,  whatever 
position  the  shield  may  take. 

Cast-iron  tunnels,  30  feet  in  horizontal  diameter,  with  a  flattened  invert,  have 
been  built  for  cross-over  roads  in  the  Great  Northern  and  City  Railway  (finished 
1903).  They  were  built,  however,  without  the  use  of  shields. 

They  are  the  only  ones  in  which  the  circular  section  has  not  been  adopted, 
except  the  small  one  constructed  at  Antwerp  in  1879,  which  was  almost  rectangular 
in  form  (see  Fig.  14). 

The  size  and  weight,1  as  well  as  the  thickness  of  the  segments  forming  the 
tunnel  rings  are  determined  more  by  practical  considerations  of  facility  of  casting, 
and  of  erection,  than  on  any  theoretical  grounds. 

The  depth  of  the  flanges  of  the  segments,  that  is,  the  thickness  of  the  lining 
from  inside  to  outside,  is,  however,  a  dimension  which  must  have  some  relation  to 
the  diameter  of  the  tunnel  ;  which  proportion  will  be  less  or  more,  according  to  the 
nature  of  the  surrounding  material  :  which,  if  made  unnecessarily  large,  will  seri- 
ously affect  the  cost  of  the  tunnel  by  increasing  the  amount  of  excavation  required  ; 
and,  if  too  small,  will  permit  of  distortion  of  the  lining  from  a  true  cylindrical  form 
and  consequently  of  the  setting  up  of  bending  movements  round  the  joints. 

The  following  table  will  show  the  proportion  of  the  flange  depth  to  the  external 
diameter  of  the  tunnel  of  some  typical  recent  works  : — 


! 

External                  Depth  of 
Diameter.                   Casting. 

Proportion 
of  Depth  of 
Casting  to 
Diameter. 

TUNNELS  IN  LONDON  CLAY:  —                                          ft. 

in. 

in. 

City  and  South  London  Railway,  Small  Tunnel, 

(1886)         

10 

10a 

43 

•033 

Ditto     Station  Tunnel  (1899)       .       .             .      . 

22 

1U4 

6 

*t 

7| 

V/*JtJ 

•029 

Ditto     Cross-over  Tunnel  (1899)  

25 

0 

9 

•030 

Ditto     Large  Station  Tunnel  (1899)       .      .      . 

32 

0 

12 

•031 

Central  London  Railway,  Small  Tunnel  (1896)  . 

12 

6 

4J 

•032 

Ditto     Station  Tunnel       

22 

6 

7| 

•029 

Waterloo  and  City  Railway,  Small  Tunnel  (1894) 

13 

0 

5i 

•033 

TUNNELS  IN  WATER-BEARING  STRATA  :  — 

Baker  Street  and  Waterloo  Railway  (portion) 

12 

9| 

4J 

•031 

Mersey  Tunnel  (1888)  ;       11 

0 

6 

•045 

Greenwich  Tunnel  (1899)  12 

9 

6 

•039 

Glasgow  Harbour  (1890)    

17 

0 

6 

•029 

Hudson  River  Tunnel  (1879)        

19 

6 

9 

•033 

Blackwall  Tunnel  (1891)    

27 

0           /        10 

f     -030 

1        12 

\     -037 

St.  Clair  Tunnel  (1888)      

21 

0                                *7 

•027 

Rotherhithe  Tunnel  (1904)      .... 

30 

0 

14 

•038 

For  tunnels  in  the  London  Clay  an  average  depth  of  casting  equal  to  '031  of 
the  external  diameter  of  the  tunnel  has  proved,  therefore,  satisfactory  for  tunnels 
up  to  32  feet  in  outside  diameter,  while  those  in  variable  water-bearing  strata, 

1  For  these  details  in  various  tunnels  see  Tables  at  end  of  chapter. 

50 


CAST-IRON    LINING    FOR    TUNNELS 

although,  as  might  be  expected,  showing  less  uniformity  in  the  proportions,  have 
an  average  depth  of  cast  iron  equal  to  0'035  of  the  external  diameter,  the  size  of 
the  tunnels  ranging  from  10  feet  to  30  feet  outside  measurement.  In  the  most 
recent  large  tunnels,  however,  the  thickness  of  the  iron  lining  bears  a  higher 
proportion  than  in  the  earlier  ones  to  the  diameter. 

The  width  of  a  tunnel  ring  depends  in  the  first  place  on  the  diameter  of  the 
tunnel.  The  greater  the  diameter  of  the  tunnel  the  greater  the  length  of  stroke 
which  can  be  given  to  the  shield  rams  without  making  the  shield  too  long  in  propor- 
tion to  its  diameter  (and  of  course  every  inch  added  to  the  width  of  a  ring  adds 
two  to  the  length  of  the  shield),  and  when  a  tunnel  is  built  with  either  horizontal 
or  vertical  curvature,  it  is  very  important  to  keep  the  shield  as  short  as  possible, 
in  order  to  facilitate  the  driving  of  it. 

In  small  tunnels  too  the  impossibility,  by  reasons  of  lack  of  space,  of  using 
mechanical  means  for  handling  the  segments  limits  their  size  by  the  weight  the 
miners  can  conveniently  handle  ;  in  larger  tunnels,  where  mechanical  erectors  are 
used,  this  consideration  has  no  weight,  and  the  width  of  the  segments  is  settled 
on  other  grounds. 

Speaking  generally,  with  any  given  depth  of  flange  from  back  to  front,  an 
increase  in  the  width  of  a  segment  beyond  a  certain  limit,  means  that,  in  order  to 
secure  a  good  casting,  the  thickness  of  the  web  or  skin  of  the  segment  must  be 
increased  beyond  what  is  necessary  for  strength. 

The  practical  experience  of  the  last  eighteen  years  has  fixed  1  foot  6  inches 
to  1  foot  9  inches  as  a  satisfactory  width  for  tunnel  segments  in  tunnels  of  all  sizes  ; 
one  important  work  only,  the  Blackwall  Tunnel  (to  which  must  soon  be  added 
the  Rotherhithe  Tunnel)  is  built  with  segments  greatly  exceeding  these  dimensions. 

Even  in  tunnels  as  large  as  that  at  Rotherhithe,  where  the  ordinary  rings  are 
30  inches  wide,  it  has  been  found  advisable  to  reduce  these  on  the  curved  portions 
of  the  tunnel  to  18  inches. 

In  the  numerous  railway  tunnels  of  10  feet  6  inches  to  13  feet  6  inches  diameter 
in  London  Clay,  the  width  of  a  ring  is  now  invariably  20  inches  ;  in  the  larger  tunnels 
for  stations  in  the  same  material  from  20  feet  to  30  feet  in  diameter,  18  inches  is 
the  width  always  adopted. 

In  certain  cases,  when  the  small  underground  passages  in  the  stations  of  the 
Central  London  Railway  were  constructed  in  iron  without  a  shield,  the  rings  were 
made  of  special  tapered  castings  where  the  curves  of  the  tunnel  were  of  small  radius  : 
but  these  cases  were  few  in  number,  and  from  their  character  of  small  importance. 

In  the  Greenwich  Footway  Tunnel,  where  there  are  two  vertical  curves  of  800 
feet  radius  for  a  distance  in  each  case  of  60  feet,  special  tapered  rings  were  specified, 
and,  the  tunnel  being  in  water-bearing  strata,  undoubtedly  made  for  improved 
tightness  in  the  joints.  But  the  confusion  likely  to  occur  in  actual  tunnel  work 
from  the  employment  of  differing  patterns  of  segments  varying  but  little  in  dimen- 
sion the  one  from  the  other  is  to  be  deprecated.  When,  as  at  Greenwich,  it  is  speci- 
fied that  the  segments  of  adjoining  rings  shall  break  joint,  a  double  set  of  patterns 
is  required  for,  and  much  extra  trouble  caused  by,  these  tapered  rings. 

The  thickness  of  the  metal  in  the  web  or  plate  of  a  segment  is  decided  on 
practical  grounds.  Given  a  flange  depth  of  5  inches  or  thereabouts,  and  a  width 
of  segment  of  20  inches,  a  minimum  of  f  inch  in  the  web  is  about  the  limit  of  sound 
casting,  even  with  the  best  inspection,  when  segments  are  to  be  supplied  by  thou- 
sands, and  in  general,  in  work  in  London  Clay,  which  is  the  most  favourable  material 

51 


TUNNEL    SHIELDS 

for  tunnels  in  the  matter  of  uniformity  of  pressure,  f  inch  is  the  thickness  of 
web  adopted.  The  flanges  are  usually  made  thicker  to  allow  for  the  stress  produced 
by  unequal  bolting  up,  particularly  such  flanges  as  do  not  form  surface  joints,  but 
have  an  intermediate  cushion  of  pine  or  other  material.  Each  segment  has  usually 
at  least  one  hole  pierced  in  it,  about  1|  inches  diameter,  to  receive  the  nozzle  of  the 
grouting  hose.  In  water-bearing  strata,  these  holes  are,  after  grouting,  tapped 
and  plugged. 

The  number  of  segments  in  a  ring  is  determined  solely  by  considerations  of 
convenience  of  handling,  and  of  casting. 

In  the  City  and  South  London  Railway  (1886)  in  tunnels  of  10  feet  2  inches 
and  10  feet  6  inches  diameter  a  ring  is  formed  of  six  segments  and  a  key  piece  ; 
in  the  Waterloo  and  City  Railway,  with  rings  12  feet  If  inches  internal  diameter, 
there  are  seven  segments  and  one  key  ;  and  in  the  Greenwich  Footway  Tunnel  of 
heavier  metal,  there  are  eight  segments  and  key  to  a  ring  11  feet  9  inches  internal 
diameter. 

The  length  of  the  ordinary  or  large  segments  in  these  rings,  is  respectively 
5  feet  9  inches,  5  feet  8f  inches  and  5  feet  2  inches,  all  of  convenient  size  to  carry 
on  a  trolly  or  for  handling  by  four  men. 

In  the  larger  tunnels  about  21  feet  in  diameter,  built  in  the  London  Clay  for 
railway  stations,  the  length  of  the  segments  is  about  the  same,  5  feet  10  inches,  for 
the  same  reasons,  though  the  actual  erection  of  the  segments  is  generally  done  by 
mechanical  power.  In  this  latter  case,  each  segment  has  cast  in  the  inside  of  the 
web  at  about  its  centre  a  lug  by  which  it  can  be  attached  to  the  arm  of  the 
mechanical  erector. 

The  key,  or  closing  piece  of  the  ring,  is  generally  made  sufficiently  wide  to 
abmit  of  one  bolt  at  least  in  the  end  flanges.  In  some  cases,  a  solid  key  has  been 
used.  When,  however,  it  is  desired  to  make  the  successive  rings  of  a  tunnel  break 
joint,  it  is  necessary  to  make  the  width  of  the  key  equal  to  the  pitch  of  the  bolt  hole 
in  the  circumferential  joint  of  the  ring,  or  some  multiple  of  that  pitch,  and  to  have 
one  bolt  hole  or  more  in  its  end  flanges,  if  all  the  bolt  holes  in  adjoining  rings  are 
to  be  filled. 

At  the  Blackwall  Tunnel  a  solid  tapered  key  was  used  ;  the  later  practice  is, 
however,  to  have  a  hollow  key  with  parallel  flanges. 

This  of  course  involves  making  the  two  adjacent  segments  on  either  side  with 
flanges  to  fit  the  key,  and  hence  it  follows  that  in  every  ring  there  are  two 
segments  having  one  end  or  longitudinal  flange  which  instead  of  being  the  normal 
to  the  tangent  at  that  point  is  parallel  to  the  axis  of  the  tunnel,  passing  through 
the  centre  of  the  key. 

The  difference  between  the  two  different  flanges  is  not  very  perceptible,  even  in 
tunnels  of  small  diameter,  and  less  so  in  larger  ones,  and  consequently  the  usual 
practice  is,  in  order  to  facilitate  the  identification  of  these  segments,  to  make  them 
of  a  different  length  to  the  ordinary  ones  If  the  rings  are  to  break  joint  the  differ- 
ence in  the  size  of  the  segments  must  be  a  multiple  of  the  pitch  of  the  bolts  in  the 
circumferential  flange  or  a  multiple  of  it.  Usually  these  segments  are  made  shorter 
by  a  bolt  hole  than  the  others  (see  Fig.  30). 

The  segments  are  bolted  together  and  to  the  adjoining  rings,  the  bolt  holes 
being  usually  made  J-  inch  larger  than  the  diameter  of  the  bolts.  In  some  cases 
the  holes  have  been  cast  longer  on  the  axis  parallel  to  the  skin  of  the  segments 
than  on  the  other  (see  Fig.  26),  but  this  does  not  appear  necessary,  and,  in  the 

52 


CAST-IRON    LINING    FOR    TUNNELS 

author's  opinion,  the  size  of  the  bolt  holes  might,  if  the  segments  are  made  to  break 
joint,  be  with  advantage  made  to  fit  the  diameter  of  the  bolts  more  closely  than 
is  the  usual  practice. 

The  diameter  of  the  bolts  used  varies  from  f  inch  in  small  tunnels  to  1 1  inches 
diameter  in  the  large  tunnels  at  Blackwall. 

Grummets  are  always  specified  to  be  used  under  the  washers  where  required, 
and  in  tunnels  in  water-bearing  material  are  essential. 

In  the  Greenwich  Tunnel,  lead  washers  or  seals  were  introduced  with  very 
satisfactory  results.  Short  lengths  of  lead  pipe  were  slipped  over  the  bolt  under 
the  iron  washers  when  the  bolt  was  put  in  the  hole,  the  ends  of  which  were  bevelled 
off  to  form  a  receptacle  for  the  lead. 

When  the  nut  of  the  bolt  was  tightened  up  the  lead  pipe  was  forced  into  the 
bolt  hole  (see  Fig.  32)  filling  up  the  ends  of  the  hole  completely. 

The  exact  length  of  the  pieces  of  lead  pipe  being  once  determined,  a  water- 
tight seal  was  obtained  with  certainty  and  with  much  less  trouble  than  by  any 
other  form  of  caulking  the  holes.  The  risk  of  electrolysis  being  set  up  does  not 
appear  sufficiently  great  to  form  an  objection  to  the  use  of  this  lead  packing.  An 
objection  which  was  raised  at  the  time  to  their  use,  namely  that  the  inelasticity 
of  the  lead  might  cause  a  leaky  joint,  after  the  tunnel  had  been  subjected  to  changes 
of  temperature,  has  proved  to  be  baseless.1  The  daily  amount  of  infiltration  in  this 
tunnel,  which  is  1,200  feet  long,  and  surrounded  by  water  throughout  its  length, 
is  only  some  240  gallons. 

The  flanges  of  the  cast-iron  segments  are  in  general  made  somewhat  thicker 
than  the  web  or  skin,  and  for  convenience  of  casting  are  tapered  from  the  bottom 
to  the  top  edge. 

The  horizontal  or  longitudinal  flanges,  each  of  which,  in  a  circular  tunnel,  is 
radial  to  the  circle,  act  as  skew  backs  to  the  flange  of  the  adjoining  segment,  and  are 
on  this  account  of  more  importance  than  the  circumferential  flanges,  which  perform 
no  such  work,  and,  only  by  the  bolts  connecting  the  adjoining  rings,  distribute  in 
case  of  the  settlement  of  one  ring  some  of  its  load  to  the  next. 

So  long  as  the  horizontal  joints  of  a  ring  are  closed  so  that  the  flanges  of 
adjacent  segments  are  in  contact  for  the  entire  depth  of  each  flange,  the  ring, 
being  subject  to  pressure  from  without  only,  is  as  strong  or  stronger  at  the  joints 
than  elsewhere. 

If,  however,  by  carelessness  in  the  foundry,  or  in  the  work  of  erection  of  the 
ring,  these  joints  are  open  so  that  adjoining  segments  bear  only  on  the  extreme 
inner  or  outer  edge,  then  any  movement  of  the  ring  due  to  pressure  from  outside 
is  restrained  only  by  the  strength  of  the  bolts  holding  the  segments  together,  and 
by  them  only  if  in  the  first  instance  they  have  been  tightly  screwed  up,  a  condition 
which  is  not  always  to  be  counted  on. 

These  considerations  have  led  to  the  adoption  of  various  kinds  of  joint,  but  the 
general  opinion  in  recent  years  is  that,  both  in  dry  and  water-bearing  material  a 
horizontal  joint  formed  of  flanges  planed  or  milled  and  in  contact  for  their  entire 
depth,  save  only  for  a  caulking  space,  is  the  best. 

The  cast-iron  tunnels  of  the  Glasgow  District  Subway  were  constructed  with 
horizontal  joints  having  fillets  at  the  back,  wood  packings  being  used  between  the 
faces,  and  Mr.  Simpson,  the  engineer  of  the  work,  defended  this  design  on  the  ground 
that,  the  fillets  being  once  in  contact,  the  joint  cannot  open  inwards  by  pressure 

1  See  pages  53  and  63. 
53 


TUNNEL    SHIELDS 


j'jprixontaL    Jbirut. 


tiircumferentjlat-  Joint, 


from  outside,  and  that  any  tendency  to  open  outwards  is  prevented  by  the  solidity 
of  the  grouting  behind.  This  line  of  reasoning  appears  to  assume  that  the  bolts  in 
the  horizontal  flanges  are  not  only  capable  of  resisting  any  tendency  to  open 
inwards,  as  they  in  a  well  designed  tunnel  are,  but  also  that  they  are  perfectly 
screwed  up,  which  is  not  always  the  case.  This  type  of  horizontal  joint  has  not 
been  used  since,  and  most  engineers  are  in  accord  with  the  opinion  of  Sir  B.  Baker 
that  "  were  he  a  contractor  he  would  have  planed  joints  even  if  he  had  to  pay  the 
extra  expense  of  them  himself." 

The  system,  now  becoming  more  common,  of  breaking  joint  in  successive  rings, 
ensures  that  a  weak  place  in  one  ring  due  to  bad  fitting  does  not  coincide  with  a 

similar  point  in  the  adjacent  ones,  and 
has  the  further  advantage  that  it  checks 
at  once  any  tendency  in  the  tunnel  to 
get  out  of  shape. 

That  the  cast-iron  segments  of  a  tun- 
nel should  be  so  arranged  that  wherever 
the  key  of  a  ring  be  placed  all  the  bolt 
holes  of  that  ring  should  be  true  with 
holes  in  the  next  rings  is  now  an  accepted 
condition  of  this  class  of  work,  and  if 
this  system  be  adopted  the  exactness  of 
the  horizontal  joint  is  not  so  important  as 
when  all  horizontal  joints  are  in  a  con- 
tinuous line. 

One  advantage  of  the  horizontal  joint 
with  an  outside  fillet  and  wood  packings 
is  that,  in  water-bearing  strata,  it  is  pos- 
sible, if  leaks  show  themelves  after  the 
tunnel  is  constructed,  to  tighten  the 
joints  by  driving  in  hard  wood  wedges. 
This  was  done  in  many  places  in  the 
Glasgow  Subway,  and  the  cost  of  such 
work  in  compressed  air  is  given  as  Is.  5d. 
per  yard  of  joint,  rust  jointing  under  the 
same  conditions  costing  from  2s.  3d.  to  2s.  Qd.  per  yard. 

The  circumferential  joints  are  not  so  important  to  the  strength  of  the  tunnel 
as  the  horizontal  ones,  and  consequently  the  variations  in  their  type  are  dictated 
more  by  the  necessity  or  otherwise  of  having  a  watertight  connexion  than  by 
considerations  of  strength. 

SOME  EXAMPLES  OF  IRON-LINED  TUNNELS 
The  Central  London  Railway 

The  cast-iron  segments  used  on  the  Central  London  Railway  are  a  good  type 
of  the  tunnel  lining  used  in  the  London  Clay.  These  are  shown  in  Figs.  23,  24  and  25. 

In  the  small  tunnels  of  this  railway  the  flanges  of  the  segments  at  the  horizontal 
joints  are  unplaned,  and  between  the  segments  is  placed  a  creosoted  pine  packing, 
a  little  narrower  than  the  metal  flanges,  pierced  with  holes  for  the  bolts,  £  inch  in 
diameter,  which  hold  the  segments  together. 

54 


FIG.  23.     CENTRAL  LONDON  RAILWAY. 


Details  of  Joints  of  Cast-Iron  Lining  for  Tunnels 
11  feet  81  inches  in  internal  diameter. 


CAST-IRON    LINING    FOR    TUNNELS 

The  packing  is  made  narrower  than  the  flanges,  to  allow  of  subsequent  pointing 
-of  the  joint  with  cement. 

In  the  circumferential  joints  the  flanges  are  made  with  a  fillet  at  the  back 
{or  outside)  f  inch  wide,  and  |  inch  deep.  Immediately  within  this  fillet,  and 
behind  the  bolts,  is  placed  a  rope  of  tarred  hemp,  sufficiently  thick  to  make  a  joint 
fairly  impervious  to  moisture  when  the  rings  are  well  bolted  up. 

When  the  grout  is  being  forced  behind  the  tunnel  the  tarred  hemp  prevents 
its  return  through  the  joints  into  the  tunnel,  and  in  cases  where,  as  happens  some- 
times even  in  London  Clay,  water  is  met  with,  it  keeps  the  joint  fairly  dry  until 
the  rust  cement  permanent  jointing  which  in  such  cases  is  usually  employed  has 
time  to  set. 

In  ordinary  circumsta-nces  the  circumferential  joint  is  pointed  with  Medina 
cement. 

In  the  larger  tunnels  for  the  stations  and  cross-over  roads  on  this  railway, 
the  horizontal  joints  are,  as  shown  on  Pig.  25,  metal  to  metal,  the  surface  of  the 


FIG.  24.     CENTRAL  LONDON  RAILWAY. 
Cast-Iron  Lining  for  Tunnels  21  feet  2J  inches  in  internal  diameter. 

flanges  being  machined  smooth.  The  circumferential  joints  are  similar  to  those  of 
the  smaller  tunnels. 

The  bolt  holes  in  these  large  tunnels  are  If  inches  in  diameter,  for  1^-inch  bolts, 
and  in  the  horizontal  joints  were  staggered  ;  in  the  circumferential  joints  they  are 
arranged  so  that  the  successive  rings  of  the  lining  may  break  joint  when  required, 
as  is  the  case  also  with  the  smaller  tunnels. 

This  work  was  the  first  in  which  the  condition  that  the  rings  should  break 
joint  was  laid  down. 

In  the  small  tunnels,  1 1  feet  8 £  inches  internal  diameter,  the  rings  were  always 
built  with  continuous  horizontal  joints  and  the  key  at  the  top,  unless  for  some 
special  reason,  such  as  the  necessity  for  an  opening  in  the  side  of  the  tunnel  at 
some  particular  level,  the  segments  of  one  or  two  rings  were  swung  round  ;  but  in 
the  larger  tunnels  all  rings  were  made  to  break  joint. 

In  these  tunnels  no  special  castings  were  used  for  those  lengths  on  horizontal 

55 


TUNNEL    SHIELDS 

or  vertical  curves,  although  some  of  the  large  21  -foot  tunnels  were  built  in  five- 
chain  curves  and  one  of  the  smaller  ones  on  a  200-feet  one. 

The  necessary  packing  of  the  joints  to  keep  the  tunnels  true  to  the  curves  was 
done  in  the  case  of  small  tunnels  by  the  use  of  iron-plate  liners  which  were  inserted, 
two  to  each  segment,  in  the  circumferential  joints. 


JoLnJb  . 


FIG.  25.     CENTRAL  LONDON  RAILWAY. 
Details  of  Joints  of  Cast-Iron  Lining  of  Tunnels  21  feet  2£  inches  internal  diameter. 


In  the  two  larger  tunnels  at  the  Bank  Station,  built  respectively  on  a  five-chain 
and  300-feet  curve,  hard  wood  packings  of  graduated  thickness  were  used. 

In  the  latter  case  the  castings  on  the  outside  of  the  curve  required  packings 
T31  inches  thick. 

56 


CAST-IRON    LINING    FOR    TUNNELS 

The  Waterloo  and  City  Railway 

In  the  Waterloo  and  City  Railway  1  ordinary  tunnels,  12  feet  internal  diameter, 
the  flanges  of  the  segments  are  somewhat  differently  treated  (see  Fig.  26). 

The  horizontal  flanges  are  planed,  and  no  pine  packing  is  used,  the  joint  being 
metal  to  metal.  In  ordinary  clay  the  flanges  are  planed  for  their  entire  depth  ; 
in  water-bearing  material  a  f-inch  caulking  space  on  the  inside  of  the  flange  is  pro- 
vided, which  is  made  solid  with  rust  jointing.  This  form  of  horizontal  joint  is  the 
most  satisfactory  of  all,  provided  that  care  is  taken  that  the  flanges  are  actually  in 
contact. 

The  circumferential  joints,  so  far  as  regards  their  shape,  are  similar  to  those 
of  the  Central  London  Railway  tunnels,  the  flanges  being  made  with  a  fillet  at  the 
back,  unplaned,  so  that  packing  can  be  inserted  between  them.  The  methods  of 
making  the  joint  are,  however,  somewhat  different. 


Jotii£s. 


J7z_  water- bearing  strata. 
asicL  thje  groove,  ru^fjotjrtecL 

Jbistfc 


FIG.  26.     WATERLOO  AND  CITY  RAILWAY,  LONDON. 
Details  of  Joints  of  Cast-iron  Lining  for  Tunnels  of  12  feet   \\  inches  internal  diameter. 


Where  the  tunnels  are  in  London  Clay  a  tarred  hemp  rope  is  placed  immediately 
within  the  fillet,  a  packing  of  creosoted  yellow  deal  of  varying  thickness  as  required 
filling  the  rest  of  the  joint  to  within  £  inch  of  the  inner  face,  the  remaining  space 
being  pointed  with  neat  Portland  cement. 

In  water-bearing  strata,  the  rope  of  tarred  oakum  is  replaced  by  a  f-inch  round 
red  rubber  packing,  the  remainder  of  the  joint  being  caulked  with  rust  cement. 
The  use  of  the  rubber  had  the  effect  of  keeping  the  joint  free  from  water  until  the 
rust  jointing  was  set  hard,  but  it  is  an  expensive  material  to  use,  and,  as  is  pointed 
out  by  Mr.  Dalrymple  Hay  in  his  account  of  the  work,  no  better  than  yarn  as  a 
solid  joint  for  keeping  the  tunnel  in  shape. 

To  obviate  this  difficulty,  hard  wood  packings  were  used  in  the  joints  from 
time  to  time  to  keep  the  tunnel  in  alignment. 

It  is  also  said  that,  in  water-bearing  strata,  the  ordinary  white  wood  packings 
were  in  the  first  place  put  in  the  joints  with  the  rubber  packing  behind,  and  when 

1  Proc.  Inst.  C.E.,  vol.  cxxxix.  p.   32. 
57 


TUNNEL    SHIELDS 

it  was  necessary  to  commence  the  rust  jointing,  were  cut  out  again,  and  the  full 
depth  of  the  joint  caulked  with  rust.  It  is  not  easy  to  see  how  with  the  best 
inspection  the  complete  removal  of  the  wood  was  effected,  nor  to  understand 
how  an  open  joint  extending  behind  the  bolts  was  satisfactorily  caulked  with 
rust  afterwards. 

In  practical  tunnel  work,  it  is  fairly  well  proved  now,  that  a  rust  joint  can  only 
be  made  satisfactorily  tight  if  the  groove  to  be  filled  is  free  from  all  obstructions 
such  as  bolts  make.  The  caulking  groove  shown  in  the  horizontal  joint  in  Fig.  26 
can  with  ordinary  care  be  made  watertight  by  rust  cement  with  little  trouble  ; 
it  is  doubtful  if  the  attempt  to  caulk  the  groove  of  the  circumferential  joint  in  the 
same  figure  would  result  in  obtaining  a  tight  joint  beyond  the  bolts  ;  that  is,  the 
extra  labour  spent  in  trying  to  make  a  deeper  joint  is  probably  wasted. 

The  larger  tunnels  for  the  City  Station  of  this  line  were  similar  in  their  joints 
to  the  smaller  ones  just  described. 

A  feature  in  both  tunnels  is  that  the  bolt  holes  were  all  made  ellipsoidal  or 
slotted  in  shape,  measuring  in  the  smaller  or  ordinary  tunnels  1|  inches  long  by  1| 
inches  broad  for  f-inch  bolts,  the  longer  axis  being  parallel  to  the  edge  of  the  flange. 

The  design  of  these  two  tunnels  typifies  fairly  well  the  construction  of  cast-iron 
tunnels  in  solid  fairly  dry  material,  but  tunnels  constructed  in  water-bearing  strata 
or  in  material  of  varying  density  require  a  more  careful  system  of  caulking  than 
those  just  described,  not  only  on  account  of  the  necessity  of  making  the  joints 
watertight,  but  also  on  account  of  the  irregularity  of  the  pressure  which  they  may 
have  to  resist,  and  in  consequence  greater  care  is  given  to  the  jointing  of  the 
segments  in  such  material  than  when  the  tunnels  are  in  an  almost  watertight  bed 
like  London  Clay. 

The  Baker  Street  and  Waterloo  Railway 

The  Baker  Street  and  Waterloo  *  Railway  passes  under  the  River  Thames  at 
Charing  Cross  in  two  tunnels,  12  feet  in  internal  diameter,  and  the  iron  lining  of 
these  tunnels  shows  in  the  details  of  the  flanges  an  interesting  compromise  between 
the  type  of  flange  generally  adopted  for  tunnels  in  London  Clay,  and  that  used  in 
most  subaqueous  tunnels. 


FIG.  27.     BAKEB  STKEET  AND  WATERLOO  RAILWAY,  LONDON. 
Details  of  Joints  of  Cast-Iron  Lining  for   Tunnels   12  feet  in  internal  diameter. 

The  section  of  cast-iron  tunnel  lining  shown  in  Fig.  27  was  used  in  the  part  of 
the  railway  under  the  River  Thames,  the  material  passed  through  being  in  part 
clay,  and  in  part  gravel  of  a  very  open  character,  with  very  little  sand,  so  that  the 
tunnel  was  for  some  distance  driven  under  the  worst  possible  conditions.  Satis- 

1  Proc.  Inst.  C.E.,  vol.  cl. 
58 


CAST-IRON    LINING    FOR    TUNNELS 

factory  results  were  obtained  by  the  adoption  of  the  pattern  of  joints  figured,  under 
a  head  of  water  of  about  60  feet. 

The  horizontal  flanges  were  made  with  machined  faces,  but  instead  of  the 
caulking  groove  being  about  |  inch  deep  on  the  inside  of  the  joint,  it  was  carried 
back  round  the  bolt  holes  to  enable  the  rust  cement  to  be  packed  round  the  bolts. 
The  form  of  circumferential  joint  ultimately  adopted  is  that  shown  in  the  figure, 
machined  joints  having  been  first  used.  The  circumferential  flanges  were  not 
planed,  and  between  the  flanges  a  creosoted  pine  packing,  cut  as  shown  in  the  two 
circumferential  joints,  shown  to  have  a  packing  space  round  the  bolts.  The  pack- 
ings were  made  slightly  tapered,  being  thicker  on  the  outside  than  inside,  generally 
|  inch  as  compared  with  |  inch.  When  the  rings  were  compressed  together  by  the 
pressure  of  the  shield,  a  watertight  joint  was  thus  ensured,  even  in  cases  where  the 
joints  were  uneven,  until  the  caulking  was  complete,  and  the  joint  made  perman- 
ently secure. 

In  this  tunnel  the  cast-iron  rings  are  only  1 8  inches  wide  instead  of  the  20  inches 
usual  in  tunnels  of  this  size. 

The  bolts  are  |  inch  diameter,  in  l|-inch  circular  holes. 

The  thickness  of  the  metal  in  the  skin  or  web  of  the  castings  is  |  inch,  which 
for  tunnels  in  such  a  situation  is  somewhat  thin. 

The  machined  flanges  were  painted  before  being  put  together  with  a  mixture 
of  red  lead  and  Stockholm  tar. 

In  these  tunnels  no  grouting  holes  were  provided  in  the  lower  segments  of 
the  rings,  the  idea  being  doubtless  that  at  the  bottom  of  the  tunnel  they  were 
unnecessary,  as  the  grout  if  put  in  at  the  sides  of  the  tunnel  would  always  run 
down  to  the  invert. 

This  is  no  doubt  correct  in  clay  and  similar  beds,  and  in  tunnels  in  water- 
logged material  it  is  of  advantage  to  reduce  the  number  of  grout  holes  as  much  as 
possible,  as  they  require,  after  grouting  has  been  done,  and  before  the  air  pressure 
in  the  tunnel  is  taken  off,  to  be  tapped,  and  closed  with  a  plug. 

But  under  ordinary  circumstances  anything  which  increases  the  number  of 
different  patterns  of  segments  is  to  be  deprecated,  and  in  open  water-bearing  ballast, 
and  when  working  under  air  pressure,  the  grouting,  to  be  efficacious,  should  be  blown 
in  at  as  many  places  as  possible. 

The  Blackwall  Tunnel  under  the  Thames 

In  the  Blackwall  Tunnel  (189 1),1  which  is  a  circular  iron-lined  tunnel  of  27 
feet  external  diameter,  and  is  built  for  the  most  part  in  water-bearing  strata,  the 
crown  of  the  tunnel  being  for  some  distance  within  5  feet  of  the  bed  of  the  River 
Thames,  and  its  invert  for  a  distance  of  1,200  feet  some  80  feet  below  Thames  high 
water,  the  whole  of  the  flanges,  horizontal  and  circumferential,  of  the  segments 
are  planed  for  their  full  depth,  except  only  on  a  2-inch  caulking  groove  on  the 
inside  edge.  No  packing  is  used  in  the  joints,  nor  any  painting  or  smearing  with 
red  lead  ;  but  sometimes  when  a  web  joint  was  found,  owing  to  imperfect  fitting 
of  the  flanges,  soft  lead  wire  was  caulked  into  the  groove  to  keep  the  water  from 
the  rust  cement  until  this  had  set. 

Two  sections  of  lining  were  used  as  shown  in  Figs.  28  and  29,  the  heavier  con- 
sisting of  segments  12  inches  deep  over  all,  with  webs  2  inches  thick,  and  flanges 

1  Proc.  Inst.  C.E.,  vol.  cxxx.  p.  i^    £0 

59 


TUNNEL    SHIELDS 

proportionately  thicker  ;  and  the  segments  of  the  lighter  pattern  being  10  inches 
deep,  with  IJ-inch  webs. 

The  first  section  was  used  under  the  river  and  in  the  deeper  parts  of  tunnel, 
and  the  second  for  a  short  length  where  the  tunnel  was  nearer  the  surface. 

In  both  patterns  the  rings  are  2  feet  6  inches  wide,  each  ring  consisting  of 
fourteen  segments,  and  a  solid  tapered  key  about  6  inches  wide.  Each  segment 
is  6  feet  long  (on  the  outside  measurement). 


I'/l  -INCH       CAST  I  NwS  . 


2  -   INCH       CASTINGS  . 


FIG.  28.     BLACKWALL  TUNNEL,  LONDON. 
Cross  Section  of  Tunnel,  showing  two  patterns  of  Cast-Iron  Lining  Employed. 

The  segments  were  not  made  to  break  joint,  which,  with  a  solid  key,  would  not 
have  been  possible  to  do  without  leaving  some  blank  bolt  holes.  The  use  of  a 
tapered  key  has  some  advantage  in  case  of  fitting,  but  the  almost  universal  prac- 
tice at  present  is,  as  stated  above,  to  arrange  the  segments  of  a. ring,  so  that  all  the 
bolt  holes  in  the  circumferential  flange  will  fit  the  bolt  holes  of  the  adjoining  one 
even  if  the  two  rings  break  joint.  To  do  this  a  hollow  key,  the  width  of  which 

60 


CAST-IRON    LINING    FOR    TUNNELS 

is  not  less  than  the  pitch  of  the  circumferential  bolts,  and  if  more,  a  multiple  of 
the  pitch,  is  necessary. 

The  rust  jointing  used  in  the  joints  is  mixed  in  the  proportion  of  J-  pound  of 
sal  ammoniac  to  every  100  pounds  of  iron  filings. 

The  grout  holes  in  the  castings  are  closed  with  screw  plugs  when  the  grouting 
is  done,  and  are  made  tight  with  red  lead  and  grummets. 

The  fixing  of  these  and  the  caulking  of  the  joints  followed  as  soon  as  con- 
venient after  the  erection  of  the  rings,  the  immense  saving  of  air  resulting  from 


CROSS        SECTION 


LONGITUDINAL        SECTION. 


DETAIL  OF  KEY. 


SECTION  . 


DETAIL  OF 


LONGITUDINAL 

FIG.  29.     BLACKWALL  TUNNEL,  LONDON. 
Details  of  Joints  of  Cast-Iron  Lining. 

making  the  tunnel  joints  tight  making  it  to  the  contractor's  interest  to  lose   no 
time  in  completely  finishing  the  caulking  making  the  tunnel  watertight. 

Observations  were  taken  on  the  rigidity  of  the  rings  when  the  segments  were 
put  together  and  well  bolted  up.  It  was  found  that  the  rings  of  heavier  section, 
each  of  which  weighs  14  tons  16  cwt.,  did  not  when  set  up  singly  on  the  surface  of 
the  ground  with  only  their  own  weight  to  support,  deflect  more  than  2  inches.  The 
same  rings  when  fixed  in  the  tunnel  at  a  depth  of  80  feet  flattened  to  the  extent 
of  4  inches,  that  is  their  horizontal  diameter  exceeded  the  vertical  by  that  amount. 

In  the  case  of  the  lighter  rings,  each  of  which  weighs  10  tons  10  cwt.,  the 
deflection  was  2^  and  5  inches  on  the  surface  and  in  the  tunnel  respectively. 

61 


TUNNEL    SHIELDS 

This  tunnel  lining  has  given  highly  satisfactory  results.  Excluding  the 
open  approaches,  the  tunnel  proper  is  4,464  feet  long,  of  which  3,697  feet  is 
iron-lined.  The  iron-lined  portion  contains  over  180,000  lineal  feet  of  rust 
jointing,  and  over  210,000  bolts,  and  in  addition  to  the  possible  infiltration  from 
these  weak  points,  a  certain  amount  doubtless  comes  in  from  the  brick-lined  parts 
of  the  tunnel,  767  feet  long  in  all,  yet  the  entire  quantity  of  water  pumped  from 
the  tunnel  in  dry  weather  only  amounts  to  3,500  gallons  per  day.  (This  quantity 
was  measured  in  1904.) 

The  Greenwich  Tunnel  under  the  Thames 
In  the  Greenwich  Footway  Tunnel  (1899),1  which,  like  the  Blackwall  Tunnel, 


FIG.  30.     GREENWICH  TUNNEL,  LONDON. 
Cross  Section  of  Cast-Iron  Lining. 

was  carried  out  by  the  engineers  of  the  London  County  Council,  the  details  of  the 
cast-iron  lining   differ   but  little  from  the  Blackwall  pattern.     Some  variations, 

1  Proc.  Inst.  C.E.,  vol.  cl.  p.   12. 
62 


CAST-IRON    LINING    FOR    TUNNELS 


, 
'J'&ctts,  /-to  cfn 


tiny  Apfe-. 
{Orte.  <fc  ecfcn  <segr  merit . 

£  "tAtcfr  TTf  TTasfaer. 


however,  were  made  in  details.  The  tunnel  is  11  feet  9  inches  in  internal,  and  12 
feet  9  inches  in  external  diameter,  and  for  its  whole  length  of  1,200  feet  is  in  water- 
bearing material,  and  for  1,100  feet  actually  under  the  river  the  invert  being,  at 
the  lowest  point,  about  68  feet  below  Thames  high  water  (see  Figs.  30,  31 
and  32). 

All  the  flanges  of  the  segments  were  planed  over  their  entire  surface  with  the 
exception  of  a  caulking  groove  as  at  Blackwall,  and  rust  jointing  was  of  course  used. 
In  this  work,  however,  in  all  cases  the  grooves  were 
first  caulked  with  lead  wire.      This  which  at  first 
was  considered  a  superfluous  precaution  fully  justi- 
fied its  use.     When  the  air  pressure  in  the  tunnel 
was  removed  less  than  a  dozen  places  in  the  12,000 
lineal  yards  of  caulked  joints  required  cutting  out 
and  caulking. 

The  bolts  also  were  provided  with  lead  washers 
(see  Fig.  32),  which  proved  very  efficacious  in  keep- 
ing out  water. 

All  bolt  holes  are  made  with  their  outer  edges 
bevelled  off.  When  the  bolts  were  put  in,  lead 
washers  (short  lengths  cut  from  lead  pipes)  were 
slipped  on  them  under  the  ordinary  iron  washers, 
and,  the  bolts  being  screwed  up,  the  lead  washers 
were  forced  into  the  spaces  made  by  bevelling  off 
the  ends  of  the  bolt  holes,  completely  filling  them. 
This  arrangement  has  proved  very  successful,  and 
comparatively  few  bolts  were  found  to  be  leaking 
when  the  air  pressure  was  removed. 

Each  ring  consisted  of  eight  segments  and  a 
key,  arranged  so  that  successive  rings  can  break 
joint.  Until  this  tunnel  was  built,  the  cast-iron 
rings  had  only  been  erected  in  this  way  in  the 
station  tunnels  of  the  railways  in  the  London  Clay, 
although  the  Central  London  Railway  ordinary 
11  feet  8  inch  tunnels  were  constructed  to  do  this 
when  required. 

The  Greenwich  Tunnel  is  the  first  of  its  size  x 
in  which  every  ring  has  been  made  to  break  joint 
with  its  neighbours. 

The  thickness  of  metal  in  the  segments  is 
heavier  than  usual,  the  weight  of  the  lining  being 
4  tons  2|  cwt.  per  lineal  yard  of  tunnel  as  compared  with  3  tons  lOf  cwt.  in  a 
similar  tunnel  under  the  Thames  of  the  Baker  Street  and  Waterloo  Railway 
which  is  at  nearly  the  same  depth. 

The  bolts  also  are  heavy  for  a  tunnel  of  small  diameter.  As  regards  water- 
tightness,  this  tunnel  lining  is  very  satisfactory,  the  amount  of  water  entering 
the  tunnel  by  infiltration  being  only  240  gallons  in  twenty-four  hours. 


FIG.  31. 


GEEENWICH  TUNNEL, 
LONDON. 


Detail  of  Cast-Iron  Lining. 


1  The  East  River  Gas  Tunnel,  New  York,  was  made  to  break  joint. 


TUNNEL    SHIELDS 


The  St.  Clair  River  Tunnel 

The  St.  Clair  River  Tunnel,  constructed  in  1888-90,  is  on  the  line  of  the  Grand 
Trunk  Railway,  which  it  carries  under  the  river  near  Sarnia  and  Port  Huron  on  the 
Canadian  Boundary  with  the  United  States. 

It  is  a  cast-iron-lined  tunnel,  2,000  yards  long,  and  is  almost  entirely  built  in 
a  soft  clay,  which  again  is  overlaid  by  sand  and  silt,  and  these  materials  also  were 
met  with  as  pockets  in  the  clay  at  the  tunnel  level. 

It  was  built  with  a  shield  and  compressed  air,  and  the  segments  were  designed 
to  make  watertight  joints  everywhere. 


SECTION    THRO'  KEY. 


/*— J 


FIG.  32.     GREENWICH  TUNNEL,  LONDON. 
Details  of  Cast  Iron  Lining. 


The  tunnel  is  21  feet  in  external,  and  19  feet  10  inches  in  internal  diameter. 
Each  ring  is  1  foot  6£  inches  wide,  and  consists  of  thirteen  segments  and  a  key, 
each  segment  being  about  5  feet  long  on  the  outside. 

The  details  of  the  joints  are  shown  in  Fig.  33.  For  the  horizontal  joints  the 
flanges  of  the  segments  are  planed,  but  are  not,  as  is  usually  the  case  with  machine 
castings,  built  metal  to  metal.  Between  them  is  fixed  a  white  oak  packing,  Ta(r  inch 
thick,  which,  after  being  bolted  into  its  place,  is  swelled  by  the  absorption  of  mois- 
ture from  behind  the  tunnel  lining,  and  so  makes  a  perfectly  sound  joint.  The 
machining  of  the  flanges  seems  a  somewhat  unnecessary  expense  when  the  wood 
packing  is  used.  In  the  Baker  Street  and  Waterloo  Railway,  as  just  mentioned 
(see  page  58),  a  watertight  circumferential  joint  was  made  with  a  wood  packing 

64 


CAST-IRON    LINING    FOR    TUNNELS 

between  unplaned  flanges,  which  were,  however,  subsequently  caulked.  The  life 
of  these  wood  packings  is  somewhat  uncertain,  there  being  no  data  to  go  upon,  as 
the  period  since  they  were  put  in  is  so  short. 

The  circumferential  joints    were    differently    designed.      The    faces    of    the 
adjoining  flanges  are  not  planed,  and  the  joints  are  made  tight  in  the   first  place 


z* — ^ 


FIG.  33.     ST.  GLAIR  TUNNEL,  CANADA. 
Details  of  Cast  Iron  Lining. 


by  inserting  between  the  flanges  a  layer  of  tarred  canvas,  and  subsequently  filling 
the  caulking  space  provided  on  the  inside  with  soft  lead. 

It  is  stated  that  these  lead  joints  become  absolutely  watertight,  but  the  general 
opinion  is  against  the  use  of  lead  alone  as  a  permanent  caulking  material.  The 
lead  being  entirely  non-elastic,  and  not  making  any  bond  with  the  cast  iron,  any 
movement  of  the  tunnel  after  the  joint  has  been  caulked  must  open  the  joint,  and 
cause  a  crevice  to  be  found  between  the  metals.1 

1  See  pp.  53  and  63. 

65  F 


TUNNEL    SHIELDS 

It  is  found  that  the  tunnels  under  the  Thames  at  Blackwall  and  Greenwich, 
which  being  open  to  public  traffic  are  under  daily  inspection,  invariably  leak  more 
after  the  prevalence  of  three  or  four  days'  cold  weather  than  at  other  times  ;  the 
explanation  usually  given  being  that  it  requires  that  time  for  the  reduction  in 
temperature  to  affect  the  water  in  the  beds  under  the  river. 

These  tunnels  are  rust  jointed,  and  the  leaks  observed  in  cold  weather  are 
found  regularly  in  the  same  places,  thus  showing  that  the  contraction  in  the  tunnel 
lining  caused  by  the  cold  always  takes  effect  at  the  same  points.  The  infiltration 
(for  it  is  nothing  more)  can  therefore  be  provided  for  ;  but  were  the  joints  all  made 
with  lead  packing,  it  appears  to  the  Author  that  all  movements  in  the  tunnel  lining 
due  to  temperature  changes  would  result  in  small  movements  in  an  increased 
number  of  joints,  with  consequent  increased  difficulty  of  dealing  with  them. 

The  segments  were  cast  of  a  mixture  consisting  of  80  per  cent,  of  old  wagon 
wheels  and  20  per  cent,  of  Scotch  pig. 

The  specification  as  to  error  of  size  and  weight  allowable  was  strict,  J^  inch 
being  the  permissible  error  in  castings  5  feet  long. 

The  segments  when  finished  were  heated  to  400°  Fahr.  and  then  dipped  in  tar. 

This  tunnel  was  not  grouted  outside  as  are  the  tunnels  in  the  London  Clay. 
The  lower  half  of  the  tunnel  was  made  solid  behind  by  pouring  into  the  grout  holes 
in  the  segments  by  means  of  a  funnel  liquid  Portland  cement  grout,  the  space  left 
round  the  upper  half  of  the  tunnel  by  the  removal  of  the  shield  skin  being  allowed 
to  become  solid  by  the  settlement  of  the  clay  above. 

This,  no  doubt,  was  perfectly  satisfactory  under  the  River  St.  Clair,  where  no 
property  or  buildings  could  be  affected  by  a  slight  movement  of  the  ground  ;  it 
is  not,  however,  a  method  to  be  recommended  even  in  such  a  case. 

In  this  tunnel  a  concrete  floor  is  put  in  to  carry  the  sleepers  and  rails,  as  is 
done  in  all  the  other  railway  tunnels  constructed  in  iron,  and  in  the  lower  half 
of  the  tunnel  a  lining  of  concrete  is  made,  covering  to  a  depth  of  about  an  inch 
the  segment  flanges,  and  offering  a  smooth  surface  in  case  of  any  derailment  of  a 
train. 

It  was  also  feared  that  without  some  such  protection,  the  dripping  of  brine 
from  the  refrigerating  cars  might  damage  the  metal  of  the  tunnel  lining. 

The  upper  part  of  the  tunnel  is  simply  tar  painted  and  cleaned  from  time  to 
time. 

The  actual  amount  of  pumping  necessary  to  keep  down  the  water  leaking 
through  the  tunnel  lining  amounts  to  some  22,000  gallons  daily,  which,  compared 
with  the  results  obtained  in  other  places,  appears  a  large  amount. 

The  Great  Northern  and  City  Railway 

The  Great  Northern  and  City  Railway  (1898)  connecting  Finsbury  Park  on 
the  Great  Northern  Railway  with  the  City  of  London  differs  somewhat  from  the 
other  tube  railways  in  the  London  Clay,  both  in  the  size  of  the  tunnels  and  in  the 
use  of  brickwork  as  well  as  of  cast  iron  on  the  permanent  lining  of  the  tunnel 
(see  Fig.  34). 

The  ordinary  running  tunnels  are  16  feet  in  internal  diameter,  each  cast  iron 
ring  as  originally  erected  being  1  foot  8  inches  wide,  and  comprised  of  eight  seg- 
ments and  two  key  pieces.  The  flanges  for  the  horizontal  joints  were  not  planed,  but 
between  them  was  inserted  a  wood  packing  as  in  the  Central  London  Railway 
Tunnels.  The  circumferential  flanges  also  followed  the  usual  pattern.  But  the 

66 


CAST-IRON    LINING    FOR    TUNNELS 

provision  of  two  keys,  the  one  in  the  invert  and  the  other  at  the  soffit  of  the  tunnel, 
was  a  novel  feature,  the  bottom  key  being  provided  to  allow — the  complete  tunnel 
lining  having  been  built  in  the  ordinary  way  under  shield — of  the  lower  portion 
being  removed  in  short  lengths,  and  a  brick  lining  substituted. 

This  was  actually  done,  the  lower  key  in  each  ring  being  removed,  and  so 
permitting  the  removal  of  the  adjoining  segments. 

The  two  lower  segments  on  either  side  of  the  key  were  removed,  leaving 
practically  the  upper  half  of  the  original  cast-iron  lining  in  place. 


FIG.  34.     GREAT  NORTHERN  AND  CITY  RAILWAY,  LONDON. 
Cross  Section  of  Tunnel  in  Cast  Iron  and  Brickwork. 


The  lower  half  of  the  tunnel  was  then  enlarged  by  cutting  out  round  the  excav- 
ation already  made  4  or  5  inches  more  of  clay.  This  done  a  three-ring  brick 
lining  was  built,  having  its  internal  diameter  some  8  inches  less  than  that  of  the 
original  iron  tunnel.  The  junction  between  the  brick  and  iron  was  by  means  of 
a  shoe  or  sole  plate,  the  lower  face  of  which  was  the  full  width  of  the  brick  lining. 

It  is  claimed  that  this  brick  invert  greatly  diminishes  the  noise  made  in  the 
tunnels  by  the  trains  and  the  vibration  caused  by  them  in  buildings  on  the 
surface,  but  in  any  case  vibration  caused  by  the  running  of  trains  at  a  consider- 
able depth  in  the  London  Clay,  unless  very  heavy  locomotives  are  used,  is  an 
inappreciable  quantity,  and  the  diminution  in  noise  in  a  tunnel  smooth  lined  as 
compared  with  one  in  which  the  iron  segments  are  left  bare  is  very  small. 

67 


TUNNEL    SHIELDS 


As  to  this  latter  advantage  it  may  be  pointed  out  that  the  Waterloo  and  City 
Railway  was  lined  smooth  with  concrete  through  its  entire  length  with  the  view 
of  deadening  the  sound,  but  when  this  railway  was  opened,  the  effect  of  the  expen- 
sive lining  was  found  to  be  so  small  that 
the  engineers  of  the  Central  London  Rail- 
way then  under  construction  abandoned 
the  idea  of  lining  the  tunnels,  which  was 
specified  in  the  original  contract  for  the 
construction  of  the  line,  and  the  tunnels 
of  that  railway,  excepting  of  course  the 
station  tunnels,  were  left  of  bare  cast 
metal. 

On  grounds  of  practical  improvement 
of  the  railway  this  compound  system  of 
tunnelling  does  not  appear  very  satisfac- 
tory ;  but  it  is  possible  that  some  economy 
in  construction  may  be  made  by  the  sub- 
stitution of  brick  for  cast  iron  in  cases 
where  the  price  of  cast  iron  is  higher,  and 
the  cost  of  labour  lower,  than  in  this 
country.  But  the  advantage  in  the  matter  of  cost  would  hardly  outweigh  the 
disadvantage  of  building  the  tunnel  in  two  operations,  and  so  nullifying  the  main 
advantage  of  a  cast-iron  lining  ;  that  is,  the  possibility  of  putting  in,  once  and  for 
all,  a  permanent  lining  which  at  once  attains  its  full  strength. 

Another  feature  in  this  work  presenting  some  novelty  is  the  type  of  large 
tunnels  used  for  containing  the  cross-over  roads  or  scissors  crossings  at  the  station 


FlG.  35.   ROTHEBHITHE  TUNNEL,  LONDON. 

Cross  Section  of  Cast  Iron  Lining. 


FIG.  36.     ROTHEBHITHE  TUNNEL,  LONDON. 
Details  of  Cast  Iron  Lining. 

v/here  are  junctions  between  the  running  lines.  These  tunnels,  which  were  not 
built  under  shield,  are  not  circular.  The  upper  half  is  a-true  circle  of  15  feet  radius, 
the  lower  half  being  flattened  in  the  invert,  and  the  sides  below  the  springing  line 

68 


CAST-IRON    LINING    FOR    TUNNELS 

of  the  roof  made  of  sharper  curves.     The  effect  of  this  construction  is  to  gain  greater 
width  of  tunnel  at  the  rail  level. 

The  horizontal  flanges  in  these  large  tunnels  were  planed,  and  the  joints  bolted 
up  metal  to  metal,  a  caulking  space  being  left  in  the  inside  face  of  the  casting. 


The  Rotherhithe  Tunnel  under  the  Thames 

The  Rotherhithe  Tunnel  under  the  Thames  is  now  in  course  of  construction 
(1904),  and  the  details  of  the  cast  iron  lining  designed  for  it  are  shown  in  Figs.  35, 
36,  37  and  231.     In  general  they  resemble  the  work  at  the  Blackwall  and  Green- 
wich  Tunnels,  and  the  details  of   the   flanges  are 
only  shown  here  as  this  tunnel  is  the  last  and  largest 
built  in  iron,  its  external  diameter  being  30  feet. 

The  rings  are  2  feet  6  inches  wide,  each  having 
twelve  segments  and  one  key. 

They  are  made  to  break  joint. 
The  flanges  are  similar  to  those  at  Blackwall 
and  Greenwich,  but  they  are  specified  to  be  smeared 
with  red  lead  before  being  fixed  in  position  and 
bolted  up. 


FIG.  37. 


ROTHEBHITHK  TUNNEL, 
LONDON. 

Details  of  Key. 


The  Lea  Tunnel 

A  cast  iron  tunnel,  11  feet  6  inches  in  internal  diameter,  under  the  River 
Lea,  was  built  in  1891-2,  as  a  part  of  the  additional  outfall  sewer  on  the  north  side 
of  the  Thames  carrying  the  drainage  of  London  to  the  Outfall  Station  at  Barking 
Creek.  It  passes  under  three  branches  or  arms  of  the  River  Lea,  and  for  the 


FIG.  38.     LEA  TUNNEL,  LONDON. 
Sections  of  Cast  Iron  Lining. 


greater  part  of  its  length  is  made  in  open  ballast  and  peaty  clay.  The  depth  of  the 
tunnel  beneath  the  surface  of  the  ground  is  not  great,  about  20  feet  being  the  largest 
cover  over  the  tunnel,  and  the  bed  of  the  river  being  within  8  feet  of  the  shield  face. 

69 


TUNNEL    SHIELDS 

The  iron  lining  was  therefore  made  lighter  in  section  than  the  tunnels  in 
water-bearing  strata  previously  described  (see  Figs.  38  and  39),  the  web  or  skin 
of  the  plates  being  only  |  inch  thick,  and  the  flanges  in  proportion.  The  flanges 
were  all  planed,  a  grouting  space  being  left  within.  The  caulking  was  done  with 
rust  cement. 

When  completed  the  iron  tunnel  was  lined  first  with  concrete,  made  to  cover 


FIG.  39.  LEA  TUNNEL,  LONDON. 
Details  of  Cast  Iron  Lining. 

the  flanges  to  the  depth  of  1|  inches,  and  this  concrete  was  again  lined  with  a  brick 
ring  4J  inches  thick,  the  upper  half  being  of  ordinary  pressed  bricks,  the  lower  of 
blue  bricks. 

The  sewer  when  finished  was  found  to  be  absolutely  watertight,  which,  consider- 
ing the  small  head  of  water,  some  30  feet,  on  the  invert  was  to  be  expected. 


Casting    of    Tunnel    Segments. 

The  casting  of  the  cast  iron  segments  for  tunnel  lining  does  not  differ  greatly 
from  other  foundry  work. 

The  mixture  used  for  the  Blackwall  tunnel  segments  was  made  as  under  : — 


No.   3  Pig  Iron  (English  or  Scotch) 
Hematite       ..... 
Scrap   (machinery,  chain,   etc.) 
,,       (heads  and  gates) 


cwt. 

10 

2 


20 


The  tests  employed  were  8  tons  tensile  strength  per  square  inch  of  section  ; 
and  for  transverse  strain,  a  bar  3  feet  long  between  supports,  and  2  inches  deep 
by  1  inch  wide  had  to  bear  28  cwt.  at  the  centre. 

Test  bars  were  run  twice  a  day. 

The  moulds  for  these  castings,1  as  well  as  those  used  in  the  Greenwich  Footway 
Tunnel,  were  made  in  a  patent  moulding  machine  designed  by  the  British  Hydraulic 
Company,  the  sub-contractors  for  the  material. 

This  Machine  (see  Figs.  40  and  41)  rests  on  three  main  cast  iron  girders,  9  inches  deep  by 
9  inches  wide  on  the  flanges,  supporting  the  main  cast  iron  framing,  on  whose  top  is  carried 
the  pattern. 

The  main  cast-iron  framing  is  8  feet  3  inches  by  4  feet  4  inches  by  2  feet  deep,  and  forms 
the  recess  into  which  the  sides  of  the  pattern  are  retired  previous  to  drawing  the  body  of  the 
pattern. 

1  Proc.  Inst.  of  Engineers  and  Shipbuilders  in  Scotland,  1895.  Carey  on  "  Cast  Iron  Seg- 
ments." 

70 


CAST-IRON    LINING    FOR    TUNNELS 

The  moulding  box  surrounding  the  pattern  is  7  feet  by  3  feet  4  inches  by  1  foot  8  inches 
deep. 

The  essential  feature  of  the  machine  is  that  the  main  framing  is  hinged,  and  revolves  on 
large  bearing  surfaces  turning  completely  over,  and  carrying  bodily  with  it  both  pattern  and 
mould. 


"tt/het.1 


FIG.  40.     MOULDING  MACHINE  FOR  TUNNEL  SEGMENTS  OF  THE  BRITISH  HYDRAULIC  COMPANY. 


Four  hand  wheels  24  inches  diameter  actuate  worms  and  raise  or  lower  the  flange  of  the 
pattern  to  enable  the  mould  to  leave  the  same,  the  travel  being  the  height  of  the  flange. 

The  moulding  box  is  held  down  by  two  screw  clamps,  one  at  either  end. 

The  girders  rest  on  blocks  of  brickwork,  the  H  beams  being  bolted  together  by  Cast  Iron 
distance  pieces. 

The  pattern,  which  is  made  of  mahogany,  is  brass  bound  at  the  edges. 

71 


TUNNEL    SHIELDS 

Operations  are  conducted  as  follows  : — 

The  mould  box  being  in  place,  with  the  flanges  in  position,  the  box  is  filled  with  sand  and 
the  pattern  rammed  up ;  a  cast  iron  plate  forming  a  lid  is  then  clamped  on  to  the  top  of  the 
moulding  box  and  the  whole  main  frame  with  moulding  box  and  pattern  is  turned  over  by 
means  of  an  hydraulic  crane. 


Half  Sectional  End  £levafion,. 


va&orv. 


tfuut'-'l I' 


5  '?  5  \4-fect. 

FIG.  41.     MOULDING  MACHINE  FOR  TUNNEL  SEGMENTS  OF  THE  BRITISH  HYDRAULIC  COMPANY. 


The  flanges  are  then  withdrawn  by  means  of  the  four  handles  already  described,  and  the 
clamps  holding  the  moulding  having  been  released,  the  main  frame  is  turned  back  again  by  the 
hydraulic  crane  into  its  original  position,  leaving  the  mould  ready  for  coring,  finishing  and 
receiving  the  top  part  which  has  in  the  meantime  been  moulded"  by  hand. 

The  cores  are  made  by  hand  in  the  usual  way,  and  call  for  no  special  remark. 

72 


CAST-IRON    LINING    FOR    TUNNELS 

The  facing  sand  consists  of — 

2  parts  white  rock  sand  ~| 

1  part  Belfast  red      ,,        >  with  equal  bulk  of  old  black  sand. 

1     „     Coal  dust 

The  segments  were  all  dipped  cold  into  Angus  Smith's  composition  and  kept 
in  until  the  solution  was  made  to  boil. 

The  faces  of  the  joints  were  milled,  and  extremely  accurate  results  were 
obtained. 

The  small  keys  were  moulded  by  hand. 


IRON  TUNNELS  IN  LONDON  CLAY 
QUANTITIES  PER  YARD  FORWARD. 


Internal 
Diameter. 
Feet. 

Excavation. 
C.  yds.t 

Cast  Iron. 
Tons. 

Wrot.  Iron  in 
Bolts,  etc. 

Cwts. 

Grouting. 
Sup.  yds.2 

10-5 

11-3 

2-50 

1-75 

12-00 

11-5 

13-63 

2-83 

2-00 

12-90 

11-68 

14-00 

2-85 

1-95 

13-09 

12-42 

15-70 

3-01 

1-95                         13-90 

12-58 

16-01 

3-05 

1-95 

14-05 

13-00 

17-52 

3-25 

3-00 

14-66 

15-00 

24-00 

5-70 

3-50 

17-00 

21-20 

45-16 

8-25 

6-45 

23-56 

25-00 

62-44 

11-55 

10-25                          27-75 

27-00   , 

73-40 

14-15 

10-68 

30-40 

30-00 

91-50 

20-00 

12-00                         34-00 

NOTE. — The  quantities  in  tunnels  of  the  same  diameter  on  different  railways  vary  a  little. 
For  the  most  part  the  figures  given  above  are  those  of  the  Central  London,  and  of  the  City  and 
South  London  Railways. 


1  Measured  net  sectional  area  of  outside  tunnel. 

2  Measured  net  outside  surface  of  tunnel. 


73 


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74 


Chapter    IV 
THE  GREATHEAD  SHIELD  IN  LONDON  CLAY 

THE  SHIELD — THE  ASSISTED  SHIELD — GENERAL  CONDITIONS  or  TUNNEL  WORK  IN  LONDON 
CLAY — MOVEMENT  OF  THE  EXPOSED  FACE — TYPE  OF  STATION  SHAFTS — DETAILS  OF  THEIR 
LINING — BREAK  UP  FOR  SHIELD — CITY  AND  SOUTH  LONDON  SHIELD  THE  PROTOTYPE  OF 
ALL  SUBSEQUENT  MACHINES  IN  LONDON  CLAY — DETAILED  DESCRIPTION  OF  IT — THE 
GROUTING  PAN — THE  VARIOUS  OPERATIONS  OF  WORKING  THE  SHIELD — GENERAL  OB- 
SERVATIONS— SPEED  AN  ESSENTIAL — WEDGES  FOR  BREAKING  DOWN  THE  FACE — GUIDING 
OF  THE  SHIELD — HAY'S  PATENT — COST  OF  LABOUR  EMPLOYED — GLASGOW  DISTRICT  SUB- 
WAY SHIELD — CENTRAL  LONDON  RAILWAY  SMALL  SHIELDS — EMPLOYMENT  OF  PUMPS  ON 
SHIELD — SPECIAL  SHIELD  FOR  USE  WITH  THOMSON'S  EXCAVATOR — THOMSON'S  EXCAVATOR 
— PRICE'S  COMBINED  SHIELD  AND  EXCAVATOR — GREAT  NORTHERN  AND  STRAND  SHIELDS 
— ARRANGEMENT  OF  SHIELD  RAMS  AND  REMOVAL  OF  DIAPHRAGM — GREATHEAD  SHIELDS  OF 
LARGER  DIAMETER  THAN  13  FEET — STATION  SHIELDS  OF  THE  WATERLOO  AND  CITY  AND 
CENTRAL  LONDON  RAILWAYS — GREAT  NORTHERN  AND  CITY  RAILWAY  AND  KINGSWAY 
SUBWAY,  SHIELDS — THE  SEGMENT  ERECTOR  OF  THESE  LATTER — METHOD  OF  WORKING  OF 
THE  KINGSWAY  SUBWAY  SHIELD  ON  A  FACE  HAVING  BALLAST  AT  THE  TOP — A  METHOD 
OF  SUPPORTING  A  CLAY  FACE  IN  AN  IRON  LINED  TUNNEL  OF  LARGE  DIAMETER 

AFTER  the  construction  of  the  Tower  Subway  and  of  the  Broadway  Tunnel 
in  1869,  which  works  may  be  said  to  have  proved  the  practicability  of  tunnel- 
ling by  means  of  the  modern  shield,  no  further  work  of  the  same  kind  was  done 
until  1886,  when  the  "  London  and  Southwark  Subway,"  afterwards  called  the 
"  City  and  South  London  Railway,"  of  which  Parliament  had  authorized  the 
construction  in  1884,  was  commenced. 

The  railway  connects  the  City  of  London  with  the  suburb  of  Clapham  on  the 
south  side  of  the  River  Thames,  under  which  it  passes,  and  with  Islington  on  the 
north.  It  is,  with  the  exception  of  a  short  distance  near  Stockwell,  constructed 
entirely  in  London  Clay. 

The  portion  of  this  railway  between  the  City  of  London  and  Stockwell  Station 
was  the  length  included  in  the  first  project  of  1884,  and  on  it,  for  the  first  time  on 
a  large  scale,  the  shield  designed  by  Mr.  Greathead  was  put  to  the  test.  The 
remarkable  success  of  the  undertaking  from  an  engineering  point  of  view  speedily 
resulted  in  the  application  of  the  system  to  other  enterprises  of  like  nature,  and 
to-day  tunnelling  under  shield  in  London  Clay  is  reduced,  one  may  almost  say,  to 
an  exact  system,  but  in  the  general  design  of  the  shield  for  tunnels  under  14  feet  in 
diameter,  practically  no  change  has  been  made  since  construction  of  the  first  tunnels 
of  the  City  and  South  London  Railway. 

The  later  shields  differ  somewhat  in  the  details  of  the  mechanical  appliances, 
and  there  are  now  used  shields  of  larger  diameter  than  those  of  the  South  London 
Railway,  but  the  method  of  working  them  remains  substantially  unchanged  since 
1886,  whether  the  work  to  be  done  is  in  London  Clay  or  in  less  easily  worked  material. 

75 


TUNNEL    SHIELDS 

A  description,  therefore,  of  the  shield  as  used  on  that  railway  and  of  the  method 
of  working  it  will  serve  as  a  general  introduction  to  all  subsequent  work  under 
shield,  the  variations  introduced  from  time  to  time  in  the  structure  of  the  shields 
employed  in  water-bearing  material  being  considered  afterwards. 

For  a  short  distance  the  tunnels  on  the  City  and  South  London  Railway  were 
constructed  through  water-bearing  beds  of  open  gravel,  by  employing  the  method 
known  in  somewhat  awkward  English  as  the  "  assisted  shield  "  method. 

This  method  of  working  has  since  been  employed  in  the  Glasgow  Harbour 
Tunnels,  the  Glasgow  Circular  Railway,  on  the  Waterloo  and  City  Railway  (for  a 
portion  of  the  work  in  water-bearing  strata)  and  in  other  places. 

It  consists  in  protecting  the  excavation  in  front  of  the  Greathead  shield  by  tim- 
bering of  similar  character  to,  but  lighter  than,  that  employed  in  ordinary  tunnel 
work,  and  it  is  of  course  open  to  the  objection  that  unless  very  carefully  carried  out, 
some  settlement  of  the  superincumbent  material  which  it  is  the  principal  merit  of 
the  shield  system  of  tunnelling  to  prevent,  is  almost  certain  to  occur. 

As  the  ordinary  Greathead  shield  is  not  adapted  for  work  in  loose  water- 
bearing material,  or,  indeed,  in  loose  material  of  any  kind,  either  with  or  without 
compressed  air,  it  is  better,  when  any  considerable  length  of  tunnel  has  to  be  driven 
through  such  material,  to  provide  a  more  suitable  shield,  such  as  are  now  made, 
at  the  outset  of  the  work,  and  so,  by  the  sacrifice  of  some  of  the  ease  of  working 
on  good  material  possible  with  an  ordinary  shield,  to  obtain  better  protection  and 
increased  rate  of  progress  in  the  places  where  the  material  met  with  is  bad. 

But,  as  even  in  London  Clay,  pockets  of  ballast  are  not  infrequently  met  with, 
cases  may  arise  in  the  future,  as  in  the  past,  when  the  ordinary  Greathead  shield 
requires  the  assistance  of  timber  work  in  the  face  for  short  distances,  and,  therefore, 
the  "  assisted  shield  "  is  dealt  with  in  the  next  chapter. 

Before  describing  the  working  of  the  Greathead  shield  in  the  clay,  a  short 
description  may  usefully  be  given  of  the  general  conditions  under  which  deep  level 
tunnelling  work  is  carried  on  in  London,  and  which  are  common  to  all  the  "  tube 
railways."  Under  the  greater  part  of  London,  from  Hampstead  on  the  north  to 
Kennington  on  the  south,  and  from  the  City  on  the  east  to  the  open  country  on 
the  west,  is  a  bed  of  thick  homogeneous  clay,  known  as  London  Clay.  This  is 
usually  found  from  10  to  30  feet  below  the  present  ground  level,  and  it  has  a  thick- 
ness of  generally  70  to  100  feet.  It  varies  little  in  character  ;  in  some  places  it  is 
apparently  softer  than  in  others,  judging  by  the  rate  of  tunnelling  operations  ; 
occasionally  "  backs  "  are  found  which  require  care  when  working  a  large  face, 
and  in  places,  particularly  under  old  watercourses,  the  clay  is  found  "  short  "  and 
rotten,  but  generally  speaking  it  is  a  perfect  medium  for  tunnelling  operations,  its 
entire  immunity  from  faults  or  dislocations  of  any  kind  enabling  tunnel  work  to 
be  estimated  for  without  having  to  take  into  account  so  largely  as  is  usual  in  tunnel 
contracts  possible  contingencies  of  that  nature. 

Like  other  clay,  exposure  to  the  air  has  a  disintegrating  effect  on  London  Clay, 
and  in  working  in  it  with  a  shield,  when  a  considerable  face  is  always  exposed,  the 
rate  of  advance  is  an  important  point  on  this  account  ;  but  a  more  important  reason 
for  speed  in  tunnelling  is  the  fact  that  the  clay  when  laid  bare,  as  in  the  face  of  a 
tunnel,  actually  moves,  being  pushed  forward  by  the  pressure  behind  it,  and  that 
the  disturbance  thus  started  extends  to  the  surface  of  the  clay,  and  affects  the 
heavy  buildings  which  everywhere  cover  it  in  London.  Cases  have  come  under 
the  author's  own  observation  of  small  movements  in  houses  in  the  street  where 

76 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


tunnels  were  being  driven,  but  considerably  ahead  of  the  working  face,  which  were 
undoubtedly  caused  by  the  tunnelling  operations.  These  cases  occurred  during 
the  driving  of  small  tunnels  11  feet  8  inches  in  diameter  ;  in  the  case  of  large  21  feet 
tunnels  (station  tunnels)  some  movement  is  usually  caused,  but  in  these  cases  any 
disturbance  of  the  strata  is  usually  first  started  in  the  construction  of  the  "  break- 
up "  for  the  shield  chamber  in  which  the  shield  is  erected,  and  this  being  some 
25  feet  in  internal  diameter,  involves  the  construction  of  an  ordinary  timbered 
length.  Of  course  the  amount  of  damage  caused  by  tunnelling  under  shield  is, 
with  ordinary  precautions  in  the  working,  very  small  indeed  as  compared  with  the 
disturbance  caused  in  similar  conditions  by  the  older  method  of  tunnelling  with 


! 

I 

Rails  • 

P/al-form 

'  J^ 

i 

j 

FIG.  42.     CENTRAL  LONDON  RAILWAY. 
Plan  of  Marble  Arch  Station  Tunnels. 

timbered  excavation  and  permanent  brick  lining,  but  some  movement  always  takes 
place. 

This  movement  in  the  clay  is  usually,  indeed,  so  small  that  its  effects  count 
for  little  in  the  cost  of  the  tunnel  work.  When,  however,  the  tunnels  are  driven 
under  or  near  buildings  of  especial  weight,  or  of  importance  from  the  historical  or 
aesthetic  point  of  view,  or  by  their  construction  specially  likely  to  be  affected  by 
any  movement  of  the  subsoil,  compressed  air  at  a  moderate  pressure  is  usefully 
employed.  When  the  Central  London  Railway  was  driven  under  the  Holborn 
Viaduct,  the  tunnels,  11  feet  6  inches  in  diameter,  running  under  the  viaduct  and 
its  approaches,  supported  on  light  piers  for  their  entire  length,  the  employment  of 
compressed  air  at  a  pressure  of  about  15  pounds  per  square  inch  avoided  any  risk 
of  damage  in  a  structure  little  calculated  to  withstand  any  uneven  settlement  of 
its  foundations. 

77 


TUNNEL    SHIELDS 


Where  the  same  railway  was  constructed  in  front  of  the  Mansion  House,  the 
Bank  of  England,  and  the  Royal  Exchange,  all  buildings  of  importance,  and  in  the 
case  of  two  of  them  of  great  weight,  the  same  precautions  were  adopted  in  driving 
the  tunnels  of  the  Bank  Station,  21  feet  in  diameter,  and  built  on  300  and  330  feet 
curves  respectively,  with  complete  success.  The  solid  homogeneous  character  of 
the  clay  practically  prevented  any  escape  of  air,  and  the  full  effect  of  its  pressure 
(in  this  case  about  20  pounds)  was  employed  in  holding  up  the  face,  and  the 
ungrouted  parts  of  the  tunnel. 

The  only  movement  observed  in  any  of  these  important  and  valuable  build- 
ings was  a  fissure  in  the  foundations  of  the  Mansion  House,  which,  however,  was 
probably  not  caused  by  the  tunnels  80  feet  down  in  the  clay,  but  by  some  of  the 
works  of  the  upper  station  only  a  few  yards  below  the  surface  of  the  street. 

Water  is  never  met  with  in  the  London  Clay,  and  the  little  that  is  sometimes 
tapped  in  sinking  the  shafts  which  give  access  to  the  tunnels,  through  the  superin- 
cumbent made  ground  (the  accumulated  debris  of  centuries  amounts  on  the  average 
to  a  thickness  of  10  feet  all  over  the  City  of  London)  and  gravel,  is  easily  dealt  with 

without  the  use  of  compressed 

01,      ,  ...       „-....         M       air.     It  is   only  in   the   case  of 

•Street-  W   Star/on   Offices  W  J 

large   pockets  of    gravel,    or    or 

gravel  deposits  in  some  depres- 
sion of  the  clay  eroded  in  pre- 
historic times  so  far  below  the 
surface  that  the  deep  level  rail- 
way tunnels  pass  through  them 
that  compressed  air  is  required 
to  keep  water  out  of  the  work- 
ings. 

The  railway  tunnels  under 
London  (and,  with  few  excep- 
tions, all  the  shield-built  tunnels 
in  London  Clay  have  been  con- 
structed for  railway  purposes)  are  laid  in  all  cases  under  the  public  streets,  and 
follow  their  windings,  the  cost  of  purchasing  the  properties  necessary  to  build  an 
"  airline  "  between  two  points  being  prohibitive,  and  Parliament  not  viewing 
favourably  the  granting  of  easements  to  a  railway  company  under  private  pro- 
perty.1 

This  fact  necessitated  the  placing  of  the  shafts  which  gave  access  to  the  tunnels 
during  construction,  and  subsequently  contain  the  lifts  and  stairways  of  the  public 
stations,  off  the  line  of  the  tunnels,  and  this  arrangement  affected  of  course  the 
method  of  initiating  and  prosecuting  the  tunnel  work. 

The  shaft  not  being  on  the  line  of  the  tunnels,  it  was  in  the  first  place 
impossible  to  erect  the  shields  for  driving  the  tunnels  at  the  bottom  of  the 
shafts,  whence  they  could  start  directly  on  their  course ;  2  and  secondly,  owing  to 
the  arrangement  of  the  stations  rendered  necessary  by  the  position  of  the  shafts 

1  The  general  arrangements    of  these    railways,   except    as   they    affect  the    tunnelling 
operations,  do  not  come  within  the  scope  of  this  work. 

2  This  was  however  done  in  the  case  of  those  railways  passing  under  the  Thames,  when 
a  temporary  working  shaft  was  sunk  through  the  bed  of  the  river  in  the  line  of  the  tunnels, 
the  cost  of  the  shaft  being  repaid  by  the  economy  of  water  carriage  so  gained. 

78 


•SecTt'on  on  /ine  flff, 

FIG.   43.     CENTEAL  LONDON  RAILWAY. 
Marble  Arch  Station  Section  in  line  A  A,  Fig.   42. 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

in  regard  to  the  lines  it  was  not  possible  in  the  great  majority  of  cases  to  place  a 
convenient  working  adit  or  tunnel  from  the  shafts  so  as  to  serve  as  a  permanent 
passage  of  the  station  later. 

Figs.  42  and  43  show  a  characteristic  type  of  station,  and  indicate  how,  in 
order  to  give  access  to  both  tunnels  from  the  shafts,  it  is  necessary  to  place  the 


FIG.  44.     CENTKAL  LONDON  RAILWAY. 
Half  Sectional  Plan  of  Cast  Iron  Lined  Shaft,  23  feet  internal  diameter. 

station  passages  about  10  feet  above  the  rail  level,  while  for  construction  purposes 
an  adit  approximately  on  the  rail  level  is  most  convenient. 

The  position  of  the  shafts,  too,  adds  greatly  to  the  difficulty  of  setting  out  the 
centre  line  of  the  tunnel.  The  base  line  by  which  the  underground  alignment  is 
started  is  usually  about  18  feet  long  (the  shafts  being  20  to  23  feet  diameter),  and 


1 
1 
1 

L. 

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L. 
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V/    V/     V/     V/      ]// 

11        W        1 

1        VI        \N 

¥-    \M    -\     V^J 

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N\     NV      A    .  ft      JL_ 

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wv/w 

V/     V/      W       W        V/ 

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i 

FIG.  45.     CENTRAL  LONDON  RAILWAY. 
Part  Elevation  of  Cast  Iron  Lining  of  Shaft  23  feet  internal  diameter. 

this  base  is  on  a  line  perhaps  100  feet  long,  generally  at  right  angles  or  thereabouts 
to  the  centre  line  of  the  tunnels,  which  usually  were  driven  a  distance  of  half  a 
mile  before  connecting  up  with  the  next  station.  In  other  words  it  is  not  usually 
possible  to  start  the  alignment  of  the  tunnel  directly  from  the  surface  centre  line 

79 


TUNNEL    SHIELDS 

itself,  but  triangulation  above  ground  and  below  is  required,  which  adds  to  the 
possibility  of  error.  The  sinking  of  boreholes  from  the  surface  of  the  street  above 
in  order  to  check  the  alignment  of  the  tunnels  beneath  is  quite  useless  for  any  exact 
observations,  and  altogether  not  worth  the  expense,  which  for  a  6-inch  hole  is  usually 
about  £1  per  foot  of  depth. 

Work  is  commenced  by  sinking  the  shafts  which  are  lined  with  cast-iron  rings 
formed  of  segments  of  varying  number,  a  30-foot  shaft  having  as  many  as  sixteen 
segments  in  a  ring  (see  Figs.  44  and  45).  The  rings  are  usually  4  feet  in  depth, 
and  are  not  made  like  the  tunnel  rings  with  a  key  to  make  the  closing  piece  of  the 
ring,  it  being  found  that,  by  excavating  a  little  wider  for  the  last  segment,  it  can 
be  got  into  its  place  without  difficulty,  other  than  the  necessity  of  an  extra  amount 


L 


JforizontaJL  Joint'. 


Vertical,  JoinC. 

FIR.  46.     CENTBAL  LONDON  RAILWAY. 
Details  of  Cast  Iron  Lining  for  Shaft  23  feet  internal  diameter. 


of  grouting  behind  it  to  fill  the  extra  excavation.  The  joints  of  these  segments 
(see  Fig.  46)  follow  the  usual  lines  adopted  in  the  tunnels.  On  the  Central  London 
Railway  both  vertical  and  horizontal  joints  were  made  with  chipping  strips  on 
the  outside,  1J  inches  wide,  and  having  for  the  rest  of  the  joint  a  caulking  space 
£  inch  thick.  The  caulking  usually  consists  of  Medina  cement.  On  some  of  the 
later  tube  railways  the  vertical  flanges  of  the  segments  have  unplaned  faces,  which 
make  a  close  joint  with  the  adjoining  segments  save  for  a  small  caulking  space, 
1  inch  deep  and  J  inch  wide  (see  Fig.  46). 

In  actual  practice,  no  leakage  of  any  importance  has  occurred  in  shafts  of 
this  class,  nor  has  any  deformation  of  the  cast-iron  lining  been  observed,  so  that 
the  joints  appear  to  do  what  is  required  of  them.  But  as  in  the  case  of  the  tunnels, 

80 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


a  planed  joint  would  cost  so  little,  and  make  so  much  more  finished  a  job,  that 
for  the  vertical  joints  it  is  to  be  recommended. 

With  the  most  ordinary  care  in  sinking  these  shafts  no  disturbance  of  the 
adjoining  buildings  is  to  be  feared.  Many  of  the  shafts  of  the  Central  London  Rail- 
way are  sunk  almost  touching  the  walls  of  adjacent  buildings  which,  however, 
have  not  been  in  the  least  affected. 

The  shafts  are  sunk  below  the  depth  necessary  to  give  access  to  the  permanent 
passages  of  the  station  which  are,  as  stated  above,  usually  some  10  feet  above  rail 
level,  in  order  to  make,  for  construction  purposes,  a  tunnel  approximately  on  the 
level  from  the  shaft  bottom  to  the  line  of  the  tunnels. 


*£°^.°ot.b^.VjK^viiV;-J/r.Vii.:^ 


FIG.  47.     CITY  AND  SOUTH  LONDON  RAILWAY. 
Station  at  Kennington  Oval. 

It  is  usual  to  sink  the  shafts  bodily  through  the  few  feet  of  loose  material 
above  the  clay  by  putting  together  on  the  surface  and  excavating  the  ground  from 
beneath  them  until  they  have  sunk  far  enough  to  reach  the  clay  level.1  Once  in 
the  clay,  the  sinking  is  carried  on  by  under-pinning,  ring  by  ring,  until  the  required 
depth  is  reached.  As  in  an  ordinary  day  shift  of  10 J  hours  the  depth  of  one  ring 
can  easily  be  excavated,  and  the  cast-iron  lining  put  in,  no  risk  of  settlement  is 
incurred. 

The  successive  cast-iron  rings  have  the  bolt-holes  in  the  horizontal  flanges 
arranged  so  that  they  can  be  made  to  break  joint.  This  is  hardly  necessary  for 
strength,  but  is  very  useful  in  placing  the  segments  at  the  level  of  the  passages  of 

1  A  bottom  ring  18  inches  deep  having  no  flange  on  the  lower  edge  is  sometimes  used  as 
a  "  cutting  edge." 

8l  G 


TUNNEL    SHIELDS 

the  underground  station  in  convenient  positions  for  their  subsequent   removal  to 
form  doorways  in  the  shaft. 

In  the  earliest  railway  built  under  the  new  conditions  the  lower  portions  of  the 
•shafts  were  built  in  brickwork  (see  Fig.  47)  ;  all  the  more  recent  shafts  have  been, 
however,  built  entirely  in  cast  iron,  and  the  necessary  openings  at  the  bottom  made 
by  removing  some  of  the  cast-iron  segments  of  the  shaft,  substituting  where 
necessary  special  castings  to  fit  the  doorways. 

When  the  first  shaft  at  a  station  from  which  it  is  proposed  to  start  a  shield  is 
sunk,  a  heading,  generally  constructed  in  cast  iron,  and  about  8  feet  in  diameter,  is 
driven  to  the  centre  of  the  tunnel  whence  it  is  proposed  that  the  actual  driving  of 
the  tunnel  should  start.  In  order  to  advance  the  work  as  rapidly  as  possible  it  is 
customary,  although  the  tunnels  in  front  of  the  shaft  are  usually  of  large  diameter 
for  the  station  platforms,  to  construct  in  the  first  place  a  shield  chamber  for  the 
erection  of  a  shield  for  the  single  line  tunnel  about  12  feet  in  diameter,  which  is 
pushed  forward  without  delay,  and  the  larger  tunnel  opened  up  later. 

In  Figs.  42  and  43  this  arrangement  is  shown  in  dotted  lines.  From  the  shaft 
A  a  working  adit  or  passage  B  is  shown  terminating  in  a  shield  chamber  C,  which  is 
constructed  of  cast-iron  rings  about  3  feet  more  in  diameter  than  the  shield  to  be 
erected  in  it.  In  one  or  two  cases  on  the  Central  London  Railway  chambers  of 
this  kind  were  constructed  in  brickwork,  but  both  in  money  and  in  the  time  spent 
in  constructing  them  they  compared  very  unfavourably  with  the  iron-lined  ones. 
From  this  chamber  starts  a  tunnel  (single  line),  and  when  this  tunnel  is  well  advanced 
a  "  break  up  "  from  it,  D,  is  started  at  the  extremity  of  the  station,  in  which  is 
erected  the  shield  for  driving  the  station  tunnel  in  the  reverse  direction  to  the 
smaller  shield.  The  shield  chamber  C  is  constructed  in  the  first  place  much  as  a 
timbered  length  in  ordinary  tunnel  work,  and  the  break-up  for  the  larger  tunnel  to 
be  constructed  round  the  smaller  one  is  necessarily  on  the  same  lines.  Some 
variations  in  detail,  however,  make  it  worth  while  to  describe  the  process  of 
constructing  this  chamber. 

When  the  single  line  tunnel  has  been  driven  past  the  end  of  the  station  where 
it  is  proposed  to  start  the  station  or  larger  tunnel  shield,  a  convenient  segment  of 
the  tunnel  lining  having  been  omitted  at  that  point  to  facilitate  matters,  a  vertical 
box-heading  or  shaft  D  (see  Fig.  48)1  is  driven  upwards  in  the  centre  line  of  the 
proposed  larger  tunnel. 

It  is  usually  4  feet  6  inches  by  3  feet  9  inches,  and  is  close  timbered  all  the  way 
up.  This  is  driven  upwards  in  3  feet  6  inch  lengths  (the  miner  working  at  the  clay 
above  his  head)  with  poling  boards  and  walings  until  within  6  feet  or  so  of  the  top, 
the  last  6  feet  being  taken  out  at  one  time  ;  the  roof  is  then  supported  by  head  trees, 
carried  on  side  trees,  which  rest  on  footblocks. 

From  the  top  of  this  shaft  a  horizontal  heading,  E,  is  driven  in  the  usual 
way  for  the  full  length  of  the  break-up  and  timbered  all  round. 

When  the  heading  E  is  completed,  the  two  central  crown  bars  consisting  of 
steel  joists  F,  F  are  got  into  the  heading  and  propped  off  stump  props,  which  are 
provided  in  the  case  shown  in  the  figures  with  a  special  hook  plate  G  which  prevents 
them  coming  in  by  the  pressure  of  the  ground  behind. 

The  two  faces  of  the  heading  are  timbered  with  creosoted  poling  boards,  wedged 
tight  from  the  stump  or  ground  props,  which  are  given  a  slight  rake  and  rest  on 

1  The  figures  48,  49,  50  and  51  are  from  drawings  prepared  by  Mr.  H.  A.  Bartlett. 

82 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


half  timber  foot-blocks  H,  being  driven  up  tight  to  take  the  weight  with  a  pair  of 
oak  folding  wedges.  The  crown  bars  are  chogged  apart  with  seven  hard  wood 
chogs  J,  J. 


The  top  weight  being  thus  taken  by  the  crown  bars  and  props,  the  side  trees  are 
knocked  away  and  the  ground  excavated  on  both  sides  of  the  heading  for  its  full 

83 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

length  to  a  distance  of  about  2  feet  6  inches.     The  top  is  timbered  with  creosoted 
poling  boards  3  feet  long,  one  end  of  each  resting  on  the  crown  bar,  and  the  other 


temporarily  propped  on  a  vertical  poling  board,  which  also  serves  to  hold  the  sides 
of  the  excavation.     Two  more  crown  bars  are  then  got  into  the  heading  and  propped 

85 


3 

02 

§1 

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o  -< 


EH 


3 1 

M  o3 

h 

•  O 

5  ** 

•  S4 
o  ? 

fa  -^ 

PH  * 


86 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

as  before  with  ground  props.  This  operation  is  repeated  until  ten  bars  are  in 
position.  Each  pair  of  bars  are  grouted  up  as  they  are  placed  in  position,  which 
is  very  important  (see  Fig.  49). 

The  two  top  cills  K,  K  are  then  got  in.  These  are  made  in  two  pieces  scarfed 
together.  The  scarf  L  is  made  as  shown  in  Fig.  50,  with  a  wrought-iron  plate  at 
the  bottom  and  a  saddle  piece  on  top,  and  the  whole  is  fixed  with  wrought-iron 


FIG.  52.     CITY  AND  SOUTH  LONDON  RAILWAY. 
The  Greathead  Shield.      Back  Elevation. 

straps  and  bolts.  These  cills  are  placed  in  front  of  the  ground  props  and  level  with 
the  foot-blocks,  one  at  each  end  of  the  break-up,  12  feet  6  inches  apart.  The  cills 
are  stretched  apart  with  stretchers  M ,  M,  and  the  weight  of  the  roof  is  then  trans- 
ferred to  the  cills  by  means  of  front  props,  N,  N,  and  wedged  up  tight  with  a  pair 
of  oak  wedges. 

As  soon  as  this  is  done  the  "  second  lift  "  is  commenced  by  excavating  a  trench 
between  the  two  centre  cill  stretchers  for  the  full  length  of  the  break-up.     Two  back 

87 


TUNNEL    SHIELDS 

props  are  then  put  in  at  each  end  of  the  trench  to  take  the  weight  of  the  cills  above 
and  wedged  up  off  foot-blocks  as  before.  The  trench  is  then  widened  out  at  both 
sides  simultaneously,  and  back  props  put  in  as  for  the  first  lift,  together  with  the 
remaining  crown  bars  which  come  below  the  top  cill.  When  the  second  row  of 
back  props  is  in  position  the  second  pair  of  cills  O,  O  are  got  in  as  before  and  stretched 
apart,  and  the  weight  of  the  top  cills  taken  by  a  row  of  vertical  props  P,  P  as  before. 
The  third  lift,  which  reaches  to  within  about  6  feet  of  the  bottom,  at  the 
centre,  is  done  in  the  same  way.  In  the  bottom  lift  the  front  props  only  are  put 
in,  and  the  clay  round  the  lower  half  of  the  break-up  is  then  trimmed  off  with  the 
aid  of  a  tramjnel  to  the  sweep  of  the  iron  lining  (see  Fig.  50). 

The  next  step  is  to  get  the  iron  lining  fixed.  The  segments  are  brought  into 
the  break-up  one  at  a  time,  and  placed  in  position  and  bolted  up,  the  rings  being 
arranged  so  as  to  break  joint.  The  whole  of  the  lining  for  the  bottom  half,  or  up 
to  springing-level,  is  got  in  and  grouted  up. 

When  the  whole  length  is  up  to  springing  level,  two  of  the  rings  are  carried 
round  and  completed,  and  the  space  between  the  outside  of  the  iron  and  the  crown 
bars  and  polings  is  filled  with  Portland  Cement  Concrete,  and  the  two  rings  are  then 
thoroughly  grouted  up,  the  remaining  rings  being  successively  carried  round,  com- 
pleted, and  concreted,  until  the  break-up  has  its  cast-iron  lining  complete. 

When  the  tunnel  lining  of  the  shield  chamber,  which  for  a  2 1-foot  tunnel  shield 
is  usually  about  25  feet  in  internal  diameter,  is  complete,  headwalls  are  built  at 
either  end  of  the  chamber  (see  Fig.  51),  sometimes  inside  the  iron  lining,  sometimes 
by  cutting  into  the  clay.  In  these  headwalls  eyes  are  turned,  in  the  one  built  round 
the  smaller  tunnel  already  existing,  in  the  other  made  large  enough  to  pass  the 
shield  when  constructed.  This  latter,  in  such  a  case  as  Fig.  51,  is  usually  bricked 
up  with  horizontal  courses  save  for  the  small  tunnel  giving  access  to  the  chamber 
from  the  shaft,  during  the  erection  of  the  shield.  Of  course,  as  the  large  shield 
advances  from  the  chamber  the  length  of  small  tunnel  lying  within  the  latter  one 
is  gradually  removed. 

Figs.  42  and  43  show  a  station  of  the  type  constructed  on  the  Central  London 
Railway  ;  Fig.  47  is  one  of  the  earlier  stations  constructed  on  the  City  and  South 
London  Railway,  and  was  the  prototype  on  which  the  later  ones  were  modelled, 
save  that  at  the  time  it  was  constructed,  it  was  considered  safer  to  construct  the 
station  tunnels  in  brickwork  rather  than  in  iron,  and  they  were  consequently  built 
in  the  usual  way  with  timbered  lengths.  The  foregoing  general  remarks  will  give 
some  idea  of  the  work  preliminary  to  starting  a  shield  in  the  London  Clay,  and  the 
details  of  the  shield  and  its  working  can  now  be  considered  at  some  length. 

City  and  South  London  Railway  Shield 1 

The  general  design  of  the  City  and  South  London  Railway  Shield  is  shown  in 
Figs.  52,  53  and  54,  and  in  the  main  follows  the  type  of  the  one  used  in  the  Tower 
Subway  shown  in  Fig.  9. 

In  later  years  modifications  have  been  introduced  to  meet  altered  conditions, 
and  notably  the  increased  size  of  tunnels  built  on  the  Greathead  system  has  neces- 
sitated the  introduction  of  platforms  or  stages  to  enable  the  miners  to  attack  the 
face  in  sections,  but  the  shield  used  in  all  tunnels  in  London  Clay  of  14  feet  diameter 
and  under  consists  of  five  principal  parts  ;  the  skin  or  cylinder,  the  cutting  edge, 

1  Proc.  Inst.  C.E.,  vol.  cxxiii.     Greathead  on  "  the  City  and  South  London  Railway  " 

88 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


the  vertical  diaphragm,  the  cast-iron  segments  carrying  the  rams,  and  the  hydraulic 
rams  for  propelling  the  shield. 

In  the  City  and  South  London  Shield,  the  skin  or  cylinder  A  of  the  shield 
(see  Fig.  53)  is  composed  of  two  J  inch  plates  rivetted  together.  The  segments 
composing  the  two  plates  are  arranged  to  break  joint,  and  of  course  all  rivets  in 
the  skin  are  countersunk.  Later  practice  has  sometimes  substituted  one  |  inch 
plate  instead  of  two  £  inch 
ones  in  shields  of  this  size, 
the  butt  joints  of  the  seg- 
ments being  covered  by  an 
outside  plate,  the  front  end 
of  such  cover  being  either 
bevelled  off,  or  the  casting  of 
the  cutting  edge  thickened 
to  protect  it,  to  avoid 
stripping  when  the  shield  is 
advancing. 

There  is  little  to  choose 
between  the  two  methods  : 
the  use  of  one  plate  only 
being  a  little  cheaper  than 
the  other. 

In  shields  of  very  large 
diameter,  the  cylindrical  skin 
is  perforce  made  up  of  several 
plates  rivetted  together. 

The  front  of  the  cylinder 
is  stiffened  by  heavy  castings, 
forming  a  complete  ring  or 
cutting  edge  B. 

The  segments  of  this 
ring,  which  vary  in  number 
in  different  shields,  are  made 
with  planed  joints  put  to- 
gether metal  to  metal,  and 
are  secured  to  the  diaphragm 
behind  them  by  bolts,  and 
to  the  skin  by  tap  bolts  (see 
Fig.  55).  In  later  shields  the 
joints  of  the  segments  are 
usually  provided  with  flanges 
back  and  front  (see  Fig.  62) 
through  which  the  segments  are  bolted  to  each  other.  This  is  doubtless  an  addi- 
tional strength,  but  if  the  segments  are  truly  made,  and  the  joints  make  an  exact 
fit,  there  should  be  no  need  for  these  bolts. 

The  actual  cutters  are  formed  of  steel  plates  C,  C  (Fig.  54)  an  inch  thick,  and 
sixteen  in  number,  which  form  a  complete  circular  knife  or  chisel  round  the  front 
of  the  cast-iron  ring  B.  They  are  bevelled  off  at  the  outside  edge  (see  Fig.  55),  so 
as  to  make  a  real  chiselling  front  to  the  shield.  They  are  attached  to  the  cast-iron 

89 


Scale. 


FIG  53.     CITY  AND  SOTJTH  LONDON  RAILWAY. 
The  Greathead  Shield.     Longitudinal  Section. 


TUNNEL    SHIELDS 


ring  by  screws  fitting  into  tapped  holes  in  the  latter,  six  screws  to  each  plate, 
the  holes  in  the  steel  plates  being  slotted  so  that  if  necessary  the  plates  can  be 
advanced  so  that  thin  sharp  ends  project  outside  the  circumference  of  the  cast 
iron. 

This  arrangement  is  devised  to  fill  the  same  purpose  as  the  skin  tapered  from 
front  to  back  in  the  Tower  Subway  shield,  namely,  the  avoidance  of  friction  when 
the  shield  was  in  movement,  particularly  in  the  case  when  the  shield  was  going 

round  a  curve. 

The  play  of  f  inch  obtain- 
able by  this  means  is  not 
enough  to  make  a  clear  way 
for  a  shield  of  the  size  under 
consideration  working  in  clay 
on  a  curve  of  five  chains  radius, 
and  the  excavation  in  front  of 
the  shield  has  to  be  cut  wider  to 
allow  of  proper  steering  of  the 
shield  in  such  a  case. 

Nearly  all  shields  for  clay 
work  are  provided  with  these 
cutting  plates,  though  they  are 
omitted  in  some  of  those  last 
constructed,  but  in  the  author's 
experience  the  occasions  on 
which  they  can  be  profitably 
advanced  are  not  frequent. 
Theoretically  it  should  be  prac- 
ticable to  set  the  cutters  to 
trim  an  opening  exactly  large 
enough  for  the  shield  on  any 
given  curve  ;  in  practice  the 
clay  is  taken  out  for  each  ad- 
vance of  the  shield  to  suit  the 
amount  of  error — to  right  or  to 
left,  upwards  or  downwards — 


which  has  to  be  corrected  in 
that  particular  "  shove."  And 
if  the  miners  in  front  have  to 
do  any  extra  trimming  of  the 
circumference  it  matters  little 
whether  that  trimming  amounts 
to  2  or  to  3  inches.  In  the 


FIG.  54.     CITY  AND  SOUTH  LONDON  BAILWAY. 
The  Greathead  Shield.     Half  Front  Elevation. 


larger  shields  for  tunnels  21,  23  and  25  feet  diameter,  the  cast-iron  or'cast-steel 
cutting  edge  is  never  provided  with  these  knives. 

In  one  way  these  cutters  are  useful.  Hard  clay  stones,  and  at  a  certain  depth 
a  hard  bed  of  stone,  are  met  with  sometimes  in  London  Clay,  and  in  case  of  the 
shield  encountering  anything  of  the  kind,  a  plate  cutting  edge  is  less  likely  to 
fracture  than  a  cast  metal  one. 

It  is  also  true  that,  steel  plates  taking  a  sharper  edge  than  cast  metal,  the 

90 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

cutters  dress  off  the  clay  round  the  edges  of  the  excavation  with  greater  ease  than 
would  the  blunter  cast-iron  ring. 


c 


Perhaps  the  term  "  cutting  edge  "  always  applied  to  the  front  of  the  shield 
cylinder  gives  rather  a  wrong  impression  of  the  function  of  the  cast-iron  ring  ;  for 
it  does  very  little  cutting  out  of  the  clay,  but  is  all  important  in  stiffening  the 


TUNNEL    SHIELDS 

edge  of  the  cylindrical  skin  (which  never  projects  less  than  1  foot  in  front  of  the 
diaphragm)  and  so  preventing  any  buckling  or  deformation  of  it. 

Immediately  behind  the  cutting  edge  and  between  it  and  the  cast-iron 
segments  carrying  the  hydraulic  rams  is  a  vertical  plate  diaphragm  D,  having  in 
its  centre  a  rectangular  door  about  6  feet  high  and  4  feet  6  inches  broad.  At 
the  side  of  this  door  are  fixed  channel  irons  E,  E,  into  which  can  be  dropped 
timbers  9  inches  by  3,  for  the  purpose  of  closing  the  opening  when  work  for  any 
cause  is  suspended. 

This  diaphragm  performs  a  double  service — it  protects  the  tunnel  against  a 
sudden  fall  of  the  face,  and  it  forms  a  very  efficient  bracing  to  the  shield. 

Behind  the  diaphragm,  the  skin  of  the  shield  is  stiffened  by  the  ram  segments 
F,  F,  which,  six  in  number,  are  bolted  together  to  the  skin,  and  also  at  the  front 
end  through  the  diaphragm  to  the  cutting  edge,  and  form  a  massive  cast-iron  frame. 
The  radial  joints  of  this  frame  are  made  with  a  hard  wood  packing  between  the 
planed  ends  of  the  segments. 

These  castings  are  about  3  feet  long,  and  beyond  them  the  outside  cylinder 
or  skin  of  the  shield  extends  back  a  further  2  feet  8  inches,  or  sufficient  to  allow 
of  space  for  erecting  a  tunnel  ring  within  it,  and  at  the  same  time  to  overlap  the 
last  ring  of  the  tunnel  already  erected. 

The  dimensions  of  the  shield  for  the  tunnel  10  feet  6  inches  in  internal  and 
1 1  feet  3  inches  in  external  diameter  with  20  inch  rings  are  :  length  over  all,  6  feet 
6  inches  ;  and  diameter  inside  the  skin,  11  feet  4J  inches.  This  last  dimension 
allows  f  inch  play  round  the  tunnel,  which  is  sufficient  clearance  for  any  but  the 
sharpest  curves.  In  some  shields,  the  advantage  of  having  plenty  of  play  in  the 
tail  of  the  shield  and  at  the  same  time  reducing  to  a  minimum  the  annular  space 
through  which  the  grout  injected  behind  the  cast-iron  lining  can  find  its  way  back 
into  the  tunnel  is  obtained  by  making  the  skin  of  the  shield  with  the  usual  amount 
of  play,  but  putting  round  the  back  edge  of  it  and  inside  a  small  beading  or  flat 
strip  which  nearly  fits  the  tunnel.  This  was  done  in  the  case  of  the  Great  Northern 
and  City,  the  Blackwall,  and  St.  Clair  Tunnel  shields.  This  beading  aids  also  in 
reducing  the  friction  when  the  shield  is  in  movement. 

The  hydraulic  rams  G,  G  (Fig.  52),  six  in  number,  for  driving  the  shield  for- 
ward are  shown  in  detail  in  Fig.  55. 

They  are  bolted  to  the  ram  castings  F,  F  already  described,  and  generally  are 
fixed  so  that  the  end  of  each  cylinder  bears  against  the  flange  of  the  casting  which 
in  turn  transmits  the  thrust  of  the  ram  to  the  cutting  edge.  The  rams  of  the  shield 
if  fixed  with  absolute  accuracy  have  their  axis  exactly  parallel  to  the  horizontal 
axis  of  the  shield,  but  with  the  best  workmanship  this  is  rarely  attained,  and  as  a 
consequence  the  thrust  of  one  or  more  of  the  rams  is  slightly  oblique  in  most  shields. 
This  causes  the  shield  to  rotate,  and  though  this  movement  is  not  a  matter  of  much 
concern  hi  the  ordinary  open  shield  for  work  in  clay,  it  is  of  importance  in  certain 
types  of  shield  for  working  in  water-bearing  strata  (see  the  Greenwich  Footway 
Shield,  page  248). 

Perhaps  this  almost  universal  movement  of  rotation  might  be  checked  by 
providing  in  the  shield  a  means  of  changing  the  line  of  thrust  of  say  two  rams, 
opposite  or  nearly  opposite  to  each  other.  This  could  be  done  by  making  the  bolt 
holes  in  the  bases  of  the  ram  cylinders,  through  which  the  bolts  pass  for  fastening 
them  to  the  cast-iron  segments,  slightly  slotted  instead  of  circular,  the  longer  axis 
of  the  slot  being  at  right  angles  to  the  axis  of  the  cylinder.  This  would  permit, 

92 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

when  the  rotary  movement  of  the  shield  commenced,  the  adjustment  of  those  rams 
to  counteract  the  general  tendency  of  the  shield. 

The  cylinders  of  the  rams  are  of  cast  steel,  with  an  internal  diameter  of  6£ 
inches.  The  hydraulic  working  pressure  obtained  by  the  hand  pumps  was  1,800 
pounds  per  square  inch,  or  a  total  driving  power  with  all  the  rams  working  of  about 
160  tons. 

They  are  provided  with  two  inlet  pipes  so  that  the  rams  can  be  drawn  back 
as  well  as  run  out,  and  this  arrangement  has  always  been  adopted  in  English  prac- 
tice ;  but  in  some  of  the  shields  used  in  Paris,  and  in  the  St.  Clair  Shield,  a  smaller 
auxiliary  hydraulic  cylinder  is  used  for  drawing  back  the  rams  (see  Fig.  109). 

The  water  for  the  rams  is  taken  from  the  small  cisterns  H,  H,  which  are  fixed 
on  brackets  on  either  side  of  the  shield,  and  is  compressed  by  the  hand  pumps  J,  J. 
These  cisterns  were  on  the  City  and  South  London  shields  unprovided  with  covers, 
with  the  result  that  dirt  and  grit  got  into  them,  and  as  a  consequence  the  battens 
in  the  pumps  suffered.  In  later  shields  they  have  been  made  with  closed  tops,  and 
provided  with  a  gauge  glass.  This  was  first  done  in  some  shield  work  in  connexion 
with  the  Blackton  Reservoir  in  Yorkshire  in  1889.1 

The  hydraulic  pumps  may  be  of  any  pattern,  the  important  point  being  that 
they  be  provided  with  a  long  removable  bow  or  handle  for  pumping,  so  arranged 
that,  when  fixed,  six  or  eight  men  can  work  at  it  effectively  at  one  time.  In  later 
shields  mechanical  compressors  have  been  introduced  with  great  advantage.  Their 
use  sets  the  men  free  to  get  on  with  other  work  while  the  shield  is  going  forward, 
and  so  accelerates  the  rate  of  progress. 

The  arrangement  of  the  pressure  pipes  and  valves  enables  the  rams  to  be 
operated  in  groups  and  singly  or  all  together,  and  the  provision  of  a  reversing 
valve  made  their  withdrawal  equally  facile.  (The  disposition  of  these  pipes  is  shown 
more  clearly  in  Figs.  68  and  69,  than  in  the  plate  of  the  South  London  Shield.) 

The  shields  constructed  during  the  last  few  years  have,  in  one  detail,  a  great 
advantage  over  this  pioneer  shield.  They  are  provided  with  sets  of  reversing  valves, 
which,  often  made  in  one  casting,  and  always  very  small  in  bulk,  can  be  placed 
on  the  side  of  the  shield,  nearly  free  from  all  risk  of  damage,  and  all  together  under 
the  hand  of  the  man  controlling  the  movement  of  the  shield  (see  Fig.  164), 
instead  of  each  pair  of  valves  being  placed  close  to  the  rams  they  control. 

The  length  of  stroke  of  the  rams  of  the  South  London  Shield  is  20J  inches,  or 
a  little  more  than  the  width  of  the  tunnel  rings.  The  piston  rods  are  3|  inches 
in  diameter,  and  terminate  in  long  cast  steel  shoes  or  crossheads  K,  K,  shaped  so 
as  to  bear  as  far  as  possible  on  the  outside  of  the  tunnel  castings  and  not  on  the 
flanges  of  the  last  ring,  against  which  they  press  when  pushing  the  shield  forward 
as  against  an  abutment. 

In  the  shield  now  under  consideration  the  rams  were  doubtless  made  six  in 
number  in  order  that  their  centres  might  coincide  with  the  horizontal  flanges  of 
the  segments  of  the  cast-iron  tunnel  lining;  that  is,  with  the  point  of  greatest  strength 
in  the  tunnel  circumference. 

The  shoes  are  strengthened  against  unequal  pressure  by  a  J  inch  plate  fixed 
to  them  with  set  screws  on  the  outside  face. 

In  ordinary  circumstances,  the  time  necessary  to  force  the  shield  forward  for 
the  length  of  one  tunnel  ring  was  ten  minutes.  This  time  has  since  been  much 
reduced  by  the  introduction  of  mechanical  pumps. 

1  Proc.  Inst.  C.E.,  vol.  xxiii.  p.  90. 
93 


OF 


TUNNEL    SHIELDS 

In  actual  work  a  platform  of  planks  is  generally  laid  across  the  shield  at  the 
level  of  the  bottom  of  the  door  to  keep  the  loose  clay  from  the  face  from  filling  the 
invert:  and,  for  the  erection  of  the  upper  segments  of  the  tunnel  lining  a  removable 
platform  which  can  be  fixed  about  the  level  of  the  axis  of  the  tunnel  is  provided. 

The  shield  as  thus  built  has  the  features  which  are  especially  necessary  for 
tunnelling  in  a  fairly  solid  homogeneous  material  like  London  Clay,  which  takes  a 
certain  time  to  expand,  or  swell  on  exposure  to  the  air,  but  which,  if  the  lining  of 
the  tunnel  follows  promptly  in  the  excavation,  can  be  worked  with  greater  security 
than  almost  any  other  material. 

The  vertical  diaphragm  is  placed  too  near  the  front  of  the  shield  for  work  in 
any  material  when  there  is  any  likelihood  of  timber  work  being  required  in  the  face 
of  the  excavation,  the  door  in  the  diaphragm  is  too  high  in  the  shield  if  there  is 
any  risk  of  an  inrush  of  water,  and  the  tail  of  the  shield  is  too  thin  and  flexible  for 
work  in  any  material  less  solid  than  the  London  Clay. 

As  will  be  seen  later  in  treating  of  shields  in  water-bearing  strata  those  features 
have  been  altered  to  suit  the  altered  conditions.  But  regarded  as  a  special  machine 
designed  for  service  under  certain  ascertained  conditions,  the  shield  used  on  the 
City  and  South  London  Railway  is  beyond  all  praise. 

It  is  only  when  attempts  were  made  to  employ  it  in  material  for  which  it  was 
not  designed,  that  difficulties  have  arisen,  and  in  such  cases  it  is  certain  that  its 
use,  though  less  effective  than  in  the  London  Clay,  in  which  it  was  designed  to 
work,  has  been  advantageous. 


The  Grouting  Pan 

Although  not  a  part  of  the  shield,  the  Greathead  grouting  pan  forms  such  an 
indispensable  part  of  the  tunnelling  plant,  and  is  so  necessary  a  complement  of 
the  shield  itself,  that  its  construction  and  use  may  be  described  here. 

In  Mr.  Barlow's  patent  of  1 864  that  engineer  proposed  to  fill  the  annular  space 
left  outside  the  tunnel  when  the  shield  moved  forward  by  injecting  liquid  grout, 
but  did  not  indicate  any  method  of  doing  it.  In  the  Tower  Subway  1  Mr.  Great- 
head  endeavoured  to  grout  up  the  outside  of  the  tunnel  with  lime  mixed  in  a  tub 
with  water,  and  injected  through  holes  in  the  cast-iron  linings  by  means  of  a  hand 
syringe.  This  was  not  satisfactory  owing  to  the  lack  of  pressure,  and  to  the  too 
great  fluidity  of  the  mixture. 

Later,  in  1886,2  Mr.  Greathead  patented  a  grouting  pan,  which  has  since  been 
in  universal  use. 

A  common  pattern  is  shown  in  Fig.  56,  which  represents  the  pan  used  on 
the  Central  London,  and  on  the  Baker  Street  and  Waterloo  Railways. 

It  consists  of  a  strong  steel  cylinder  some  2  feet  6  inches  long  and  1  foot  6  inches 
in  diameter,  having  through  its  axis  a  spindle  passing  through  stuffing  boxes  at 
either  end  and  furnished  with  handles.  On  this  spindle  are  fixed  feathers  or  paddles 
A,  A.  At  the  top  of  the  cylinder  is  a  circular  opening  B,  capable  of  being  closed 
by  a  lid  with  rubber  seatings,  and  a  valve  C,  to  which  can  be  attached  a  flexible 
pipe  connected  with  an  air  compressor.  At  the  bottom  is  another  valve  D,  to  which 
is  fitted  a  flexible  hose  ending  in  a  nozzle  which  fits  the  grout  holes  in  the  tunnel 
lining. 

*  Proc.  Inst.  C.E.,  vol.  xxiiL  p.  62.  2   Patent  No.  5221  of  1886. 

94 


THE    GREATHEAD    SHIELD    IX    LONDON    CLAY 

The  cylinder  is  also  provided  with  lugs  E,  E,  by  which  it  can  be  suspended 
from  the  tunnel  roof.  More  frequently,  however,  it  is  placed  like  a  barrel  on  a 
gantry. 

The  method  of  using  the  machine  is  as  follows  : — Through  the  opening  B,  the 
cylinder  being  partly  filled  with  water,  lime  is  introduced  by  one  workman,  an- 
other in  the  meantime  turning  the  spindle  with  the  paddles  to  mrr  the  grout,  until 
the  mixture  is  of  the  consistency  of  thin  cream.  The  lid  is  then  closed,  and 
compressed  air  at  about  70  to  80  pounds  per  square  inch  introduced  by  opening 
the  valve  C. 

One  workman  keeping  the  grout  in  motion  with  the  paddles,  the  other  applies 
the  nozzle  of  the  hose  pipe  to  the  grout  hole  in  the  casting  when  the  grout  is  to  be 
injected,  and  opens  the  valve  D.  The  grout  is  then  forced  out  of  the  cylinder, 
and  passes  through  the  hose  to  the  outside  of  the  tunnel,  completely  filling  the 
vacant  annular  space.  When  the  nozzle  is  withdrawn  from  the  grout  hole,  a  wooden 
plug  is  driven  in,  which  after  the  grout  has  set.  is  removed,  and  subsequently  the 
hole  is  pointed  with  cement  grout,  or  in  tunnels  hi  water-bearing  strata  closed  with 


FIG.  56.     THE  GROUTIXG  PAX. 


a  plug  tapped  into  the  hole.     In  a  short  time  the  lime  sets  hard,  and  forms  a  ring 
round  the  iron  at  least  as  strong  as  the  clay  which  it  replaces. 

If  carefully  done,  and  done,  too,  immediately  the  shield  has  left  the  annular 
space  exposed,  this  grouting  effectually  prevents  any  settlement  of  the  surrounding 
clay. 

Next  to  promptitude  in  carrying  out  the  operation,  the  main  essential  is  that 
the  air  pressure  should  be  sufficient,  namely  about  80  pounds  per  square  inch,  not 
merely  to  force  the  grout  into  every  crevice  or  crack  in  the  clay,  but  also  because 
only  with  a  good  pressure  can  the  grout  be  mixed  sufficiently  thick  to  ensure  fairly 
quick  setting. 

Lime  is  better  than  cement  for  use  with  this  machine,  besides  being  cheaper, 
and  it  has  been  found  that  a  mixture  of  lime  and  sand  hardly  repays  the  economy 
in  lime  effected,  the  cost  of  the  careful  mining  of  the  two  in  a  dry  state  being  more 
than  the  saving  in  material. 

A  good  test  of  the  quality  of  the  grout  is  its  heating  effect  on  the  cast-iron  lining 
of  the  tunnel.  If  after  being  grouted  the  cast-iron  segments  are  warm  to  the  touch, 
the  grouting  is  probably  satisfactory. 

95 


TUNNEL    SHIELDS 

It  is  the  usual  practice  to  provide  at  least  one  grout  hole  in  each  segment  of 
a  tunnel  ring. 

In  the  shield  as  first  designed  for  the  City  and  South  London  Railway,  the 
shoes  K,  K  (Fig.  53)  bore  directly  on  the  last  ring  of  the  tunnel  already  erected, 
and  the  annular  space  between  the  outside  of  the  tunnel  and  the  inside 
of  the  tail  of  the  shield  cylinder  was  left  open,  and  consequently  every  time 
that  the  grouting  operations  were  in  hand,  it  was  necessary  to  plug  up  this 
space  with  pugged  clay  and  sacking.  This  was  found  both  troublesome  and 
ineffective,  and  after  a  time  a  simple  arrangement  of  planks  known  as  "  grouting 
ribs  "  was  adopted,  and  with  little  variation  has  been  used  in  every  shield  since. 
The  planks,  2  inches  thick  or  thereabout,  and  six  in  number,  are  shaped  to  the 
curve  of  the  shield  skin,  and  cut  long  enough  to  form,  when  placed  one  in  front  of 
each  ram  shoe,  a  complete  ring  which,  when  held  in  position  by  the  rams  against 
the'  cast-iron  tunnel  lining,  effectually  prevents  the  escape  of  any  quantity  of  grout 
from  behind  the  lining  into  the  shield  (see  Fig.  67). 

These  grouting  ribs  are  sometimes  made  with  india-rubber  flaps  on  one  side  to 
make  a  better  airtight  joint,  and  sometimes  are  strengthened  by  making  them  of 
two  planks  with  a  flitch  or  iron  plate  between,  instead  of  only  one  single  plank. 

When  first  used,  they  were  put  in  place  by  hand  as  wanted,  and  the  rams 
pumped  out  to  hold  them  up  ;  and  when  not  required  had  to  be  taken  down  and 
put  on  one  side.  In  later  shields  the  shoes  of  the  rams  are  provided  with  threaded 
holes,  in  which  set  pins  which  fasten  the  grouting  ribs  fit  (see  Fig.  67). 

Method  of  Using  the  Shield 

The  actual  work  of  iron  tunnel  construction  with  a  shield  l  consists  of  four 
operations,  which,  however,  in  practice  overlap  each  other  in  time  as  there  is  no 
necessity  to  suspend  work  on  the  others  when  one  is  in  hand.  They  are  the 
removal  of  the  clay  in  the  face,  the  pushing  forward  of  the  shield,  the  erection  within 
the  shield  of  cast-iron  segments  forming  a  new  ring  for  the  tunnel  lining,  and  the 
grouting  up  of  this  ring  immediately  it  is  clear  of  the  shield. 

Starting  with  the  shield  in  the  position  shown  in  Fig.  57 — that  is.  with  the 
hydraulic  rams  drawn  back  in  the  cylinders  and  the  tail  of  the  shield  reaching 
over  the  last  tunnel  ring  which  has  just  been  erected  within  it  and  partly  enclosing 
the  last  ring  but  one — the  first  operation  is  to  remove  the  clay  in  the  face  for  the 
full  area  of  the  shield  face.  To  accelerate  progress,  it  is  customary  to  drive  a  box 
heading  A,  some  6  or  8  feet  ahead  of  the  shield.  This  heading  is  timbered  with 
head  and  side  trees  in  the  ordinary  way,  the  height  being  about  6  feet  and  the  width 
about  4  feet.  Work  on  this  heading  goes  on  continuously. 

It  is  usual  to  excavate  the  clay  to  the  full  size  of  the  shield,  leaving  the 
so-called  cutting  edge  of  the  shield  very  little  to  do.  The  adjustable  cutters  bolted 
to  the  cast-iron  front  ring  were  used  in  the  early  days  of  the  City  and  Southwark 
Railway,  to  increase  the  area  excavated  when  passing  round  curves,  but  in  later 
work  the  adjustable  cutters  have  been  for  the  most  part  ignored,  and  in  going  round 
curves  sufficient  play  is  obtained  by  taking  out  the  excavation  in  front  of  the  shield 
some  inches  wider  than  the  face  of  the  shield. 

1  A  good  description  of  shield  work  in  clay  is  to  be  found  in  Mr.  Dalrymple  Hay's  paper 
on  the  Waterloo  and  City  Railway  (Proc.  Inst.  C.E.,  vol.  cxxxix.),  and  in  a  paper  by  Mr.  Bart- 
lett  read  at  a  students'  meeting  at  the  Institution  in  1893. 

96 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

When  the  clay  is  excavated  for  a  distance  of  about  the  length  of  one  ring  of 
the  tunnel  lining,  and  the  invert  cleaned  up,  a  platform  of  planks  B,  B  is  laid  from 
the  door  in  the  diaphragm  to  the  heading,  and  also  across  the  invert  of  the  shield, 
to  prevent  falling  clay  from  filling  up  the  invert  when  the  shield  is  in  motion,  and 
the  last  settings  of  the  box  heading  are  knocked  out. 

Piles  C, C,  generally  about  3  feet  long  and  4  inches  square,  with  iron-shod  points, 
are  fixed  in  position  in  front  of  the  shield,  the  iron  points  being  placed  in  pockets 
cut  for  the  purpose  in  the  clay  face,  and  the  rear  ends  fixed  against  the  cast-iron 
cutting  edge.  Their  function  is  to  break  down  the  clay  in  front  as  the  shield  is 
advancing. 

The  shield  is  pushed  forward  by  pumping  water  into  the  hydraulic  cylinders 
and  so  forcing  out  the  pistons  or  rather  forcing  the  cylinders  away  from  the  pistons 


FIG.  57.     THE  GREATHEAD  SHIELD  IN  CLAY. 
Shield  ready  to  Move  Forward. 

which,  by  means  of  the  shoes  K,  K  (Fig.  53),  are  bearing  on  the  tunnel  lining  already 
erected. 

The  direction  of  the  shield  is  controlled  by  manipulating  the  supply  of  pressure 
to  the  rams.  If  the  shield  is  to  be  deflected  to  the  right,  only  the  rams  on  the 
left  hand  side  are  employed  ;  if  it  is  found  to  be  pointing  downwards,  and  requires 
raising,  the  ram  in  the  invert  only  is  used,  and  so  on. 

As  the  shield  advances,  the  piles  break  down  the  clay  face,  and  by  the  time  it 
has  gone  forward  20  inches  into  the  space  cleared  for  it,  a  considerable  amount  of 
the  clay  in  front  has  been  broken  down,  and  is  heaped  on  the  plank  platforms  in 
front  of,  and  in  the  invert  of,  the  shield. 

When  the  rams  have  driven  the  shield  the  full  length  of  their  stroke,  as  shown 

97  H 


TUNNEL    SHIELDS 

in  Fig.  58,  they  are  reversed  by  turning  the  reversing  valve  and  passing  the  water 
through  the  pipe  at  the  rear  end  of  the  rams.  When  the  rams  are  withdrawn  into 
the  cylinders  the  tail  of  the  skin  is  left  clear  for  the  erection  of  another  ring  of  the 
permanent  cast-iron  tunnel,  the  segments  for  which,  six  and  a  key  piece  to  a  ring, 
have  been  brought  up  by  trollies  to  the  shield  while  the  previously  described 
operations  were  in  hand. 

The  two  segments  forming  the  invert,  and  the  two  segments  (one  on  each  side) 
next  to  them,  are  lifted  from  the  trolley  and  placed  in  position  by  the  men  with 
the  aid  of  spanners  or  bars  slipped  through  the  bolt  holes.  They  are  then  bolted 
together  and  to  the  last  ring. 

The  two  upper  segments  and  the  key  piece  are  fixed  by  means  of  a  removable 
platform  formed  of  a  few  planks  and  placed  usually  a  little  below  the  horizontal 


FIG.  58.     THE  GREATHEAD  SHIELD  IN  CLAY. 
Shield  at  End  of  Advance. 

diameter  of  the  tunnel.  The  two  upper  castings  are  lifted  by  the  men  on  to 
the  platform,  and  thence  put  in  place,  and  to  permit  of  the  better  adjustment  of 
the  key  piece,  they  are  frequently  supported  temporarily  by  props  from  the  invert, 
and  not  bolted  up  completely  until  the  key  is  in. 

When  the  ring  is  completed,  the  grouting  ribs,  if  loose  from  the  ram  shoes, 
are  replaced  and  held  in  position  by  the  shield  rams,  and  the  ring  behind  the  shield 
should  be  at  once  grouted  up. 

The  operation  of  erecting  the  segments  is  easily  performed  by  six  men  in  the 
case  of  tunnels  of  about  10  to  13  feet  diameter  in  London  Clay,  as  the  segments 
do  not,  as  a  rule,  exceed  5  cwt.  in  weight.  The  time  occupied  should  not  exceed 
twenty-five  minutes  per  ring. 

In  the  first  City  and  South  London  tunnels  the  longitudinal  or   horizontal 

98 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

joints  were  made  with  soft  wood  packing  |  inch  thick,  and  having  holes  in  them 
for  the  bolts.  These  packings  were  about  1  inch  narrower  than  the  flanges  of  the 
castings,  to  allow  of  the  joint  being  pointed  with  Medina  cement  later. 

The  flanges  were  not  machined. 

The  vertical  or  circumferential  joints  were  made  with  chipping  pieces,  between 
which  and  the  bolts  was  placed,  at  the  time  of  erection,  a  rope  of  tarred  hemp  or 
oakum,  the  remainder  of  the  open  joint  being  subsequently  pointed  with  cement. 

The  erection  of  the  ring  being  completed,  the  cycle  of  four  operations  is  gone 
through  again ;  but  of  course  in  actual  work,  not  only  has  the  driving  of  the  heading 
continued  without  interruption  throughout,  but  the  times  of  all  the  operations 
overlap  each  other.  Thus  by  the  time  the  ring  of  segments  is  erected,  the  excava- 
tion for  the  next  advance  is  well  in  hand. 

The  substitution  of  power  for  hand  labour  in  compressing  the  water  for  the 
pumps,  and  some  other  improvements  in  the  conditions  of  work,  notably  the  better 
lighting  of  the  tunnel,  now  that  electric  light  is  available,  have  made  more  rapid 
progress  possible.  In  the  South  London  Tunnels,  however,  when  once  the  foremen 
and  men  had  become  accustomed  to  the  new  conditions  a  very  good  rate  of  progress 
was  maintained,  the  daily  advance  of  a  shield  being  for  long  periods  as  much  as 
13  feet  per  day  of  twenty-four  hours. 

Not  the  least  of  the  advantages  of  the  shield  method  of  working  in  London 
Clay  is  that  it  does  away  with  the  necessity  for  specially  skilled  labour.  The  tunnel 
bricklayer  disappears  with  the  use  of  the  iron  lining,  and  any  fairly  intelligent 
unskilled  labourer  can  learn  the  manner  of  working  a  shield  in  a  week. 

General  Observations  on  Working  the  Shield  in  Clay 

A  few  general  observations  may  be  added  to  the  foregoing  description  of 
working  the  shield. 

The  first  point  to  keep  in  view  in  working  a  shield  in  clay  under  London  is 
that  for  the  avoidance  of  subsidences  speed  is  the  great  essential.  The  longer  any 
face  of  clay  is  exposed  to  the  air,  the  greater  is  its  movement,  and  consequently, 
other  things  being  equal,  there  is  likely  to  be  more  surface  disturbance  caused  by  a 
tunnel  constructed  at  the  rate  of  three  rings  per  day  than  by  one  which  grows 
by  ten  rings  daily. 

If  properly  bolted  up,  and  grouted,  the  iron  tunnel  once  built  does  not  appear 
to  settle  afterwards,  and  this  is  borne  out  by  the  fact  that  the  comparatively  small 
surface  disturbances  which  are  sometimes  noted  in  connexion  with  this  class  of 
work  are  first  produced  not  over  the  finished  tunnel  but  ahead  of  the  shield  itself. 
The  author  has  known  some  cases  where  small  cracks  in  brickwork  have  appeared 
in  buildings  60  or  80  feet  ahead  of  the  tunnel  face. 

It  is  true  that  subsidences  caused  by  tunnelling  under  shield  in  the  London 
Clay  are  in  no  case  of  a  very  serious  character,  as  compared  with  the  disturbances 
caused  by  other  methods  of  working  ;  and  if  the  work  be  properly  carried  out  do 
not  occur  at  all,  but  the  importance  of  speed  as  a  precaution  against  settlements 
cannot  be  too  strongly  insisted  on. 

In  this  matter  of  speed  the  use  of  a  heading  is  very  important.  In  an  ordinary 
tunnel  on  London  Clay  of  about  11  feet  6  inches  diameter,  seven  rings  of  tunnel 
lining,  or  say  1 1  feet  8  inches  in  twenty-four  hours,  is  a  moderate  rate  of  progress, 
with  an  advance  heading  in  front  of  the  shield  ;  without  a  heading  three  rings  per 

99 


TUNNEL    SHIELDS 

day  is  the  best  rate   of  progress,  unless  indeed  such  a  machine  as  Mr.  Price's 
excavator  be  employed.1 

In  the  case  of  a  tunnel,  with  which  the  author  was  connected,  the  engineer 
responsible  for  the  safety  of  some  public  buildings  in  the  streets  above  refused  to 
allow  the  shield  to  be  worked  with  a  heading  on  the  ground  that  the  extra  excavation 
in  front  of  the  shield  increased  the  risk  of  subsidence.  It  is  true  that  the  area  of 
clay  exposed  at  one  time  was  larger  with  a  heading  than  without,  but  it  is  difficult 
to  believe  that  the  increased  security  given  by  a  gain  of  100  per  cent,  in  time 
obtained  by  using  the  heading  did  not  enormously  outweigh  the  possible  risk  of 
harm  due  to  the  larger  area  of  clay  exposed. 

The  size  of  the  heading  should  not  be  more  than  sufficient  for  two  men  to 
work  in,  and  care  should  be  taken  that  the  head  and  side  trees  are  well  wedged 
up.  Its  position  should,  for  convenience  of  working,  be  rather  below  than  above 
the  centre  of  the  shield. 

The  use  of  piles  or  wedges  to  break  down  the  clay  face  in  front  of  the  shield 
is,  of  course,  only  effective  when  the  central  portion  is  removed  by  having  the 
heading  cut  in  it.  The  number  of  piles  depends  on  the  ram  power  of  the  shield. 
but  a  usual  number  is  ten  or  twelve.  They  should  be  fixed  with  the  bases  on  the 
solid  cutting  edge,  and  inclined  towards  the  axis  of  the  shield  so  that  their  effect  is 
to  break  away,  as  it  were,  the  sides  and  top  of  the  heading  in  advance. 

These  piles  are  usually  made  of  oak  with  iron  points,  but  the  Australian  hard- 
wood Jarrah,  without  iron  points,  has  also  been  used  on  the  Central  London  Railway 
with  satisfactory  results. 

If  properly  applied  the  piles  reduce  the  manual  work  required  of  the  miners 
to  little  more  than  trimming  the  face  and  around  the  cutting  edge,  and  preparing 
the  invert. 

The  guiding  of  the  shield  during  each  movement  forward  does  not  offer  any 
difficulty.  As  stated  above  it  can  be  easily  deflected  to  one  side  by  employing 
only  the  rams  fixed  on  the  other  side,  and  it  is  driven  round  a  long  curve  by 
applying  the  rams  on  the  outside  of  the  circle  a  little  in  advance  of  those  on  the 
inside. 

The  important  point  to  bear  in  mind  in  nearing  the  commencement  of  a  change 
of  direction,  whether  horizontal  or  vertical,  is  to  commence  the  deflection  of  the 
shield  when  the  cutting  edge  has  reached  the  point  where  the  change  should 
commence  and  not  to  wait  until  the  tunnel  is  built  to  that  point. 

The  position  of  the  shield  is  checked  for  direction  and  level  in  a  very  simple 
manner.2 

On  the  shield  are  marked  on  the  diaphragm,  usually  on  the  upper  and  lower 
edges  of  the  opening  forming  the  door,  saw  marks  which  indicate  the  vertical  centre 
line  of  the  shield.  This  centre  line  should,  when  the  tunnel  is  straight,  line  up  with 
two  plumb  lines  fixed  by  the  engineers  on  the  centre  line  of  the  tunnel  already  com- 
pleted, and  which  centre  line  is  daily  carried  forward  by  theodolite  as  the  shield 
advances. 

When  going  round  a  curve,  either  a  series  of  marks  corresponding  to  the  off- 
sets to  the  tangent  for  short  lengths  of  the  curve  are  marked  off  to  the  right  or 
left  of  the  centre  saw  mark  on  the  diaphragm,  or  conversely  the  rearmost  of  the 

1  For  the  remarkable  results  obtained  with  this  machine,  see  page  113. 

2  For  descriptions  of  the  method  of  setting  out  the  main  lines  of  tunnels,  see  Proc.  Inst. 
C.E.,  vol.  cxxxix.  p.  27,  and  vol.  cl.  p.  78. 

TOO 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

two  plumb  bobs  in  the  tunnel  is  moved  along  a  strip  of  wood  fixed  across  the  upper 
part  of  the  tunnel,  and  similarly  divided. 

When  working  on  a  curve,  the  practical  miner  in  charge  of  the  shield  gang 
generally  can  keep  his  shield  correct  in  the  daily  interval  between  the  engineer's 
visits  for  the  purpose  of  checking  the  line,  by  measuring  the  advance  of  the  shield 
with  two  slips  of  wood,  the  one  used  on  the  inside  of  the  curve  equal  in  length  to 
the  width  of  a  single  ring,  the  other  used  on  the  outside  of  the  curve  equal  to  the 
increased  length  of  tunnel  each  ring  must  occupy  there. 

Mr.    Dalrymple  Hay  has    patented   a   system   of  directing    the  shield    by 

means  of  guide  rods,  or  graduated  bars,  which  can   be    attached   to   the    shield 

on  either  side,  and  which    advance  with  it,  the  amount   of  the   advance  being 

.registered  by  the  readings  of   the   graduated   scale   with   reference   to   two   zero 

points  fixed  at  the  side  of  the  tunnel  exactly  opposite  to  each  other. 

When  the  tunnel  is  straight  the  readings  of  two  equal  scales  so  fixed  should  be 
the  same. 

When  the  shield  is  going  round  a  curve,  the  divisions  in  the  scale  are  made 
larger  on  the  guide  rod  on  the  outside  of  the  curve  than  on  the  other,  the  difference 
being  proportionate  to  the  difference  of  the  length  of  the  radii  of  the  curves 
described  by  the  two  sides  of  the  tunnel. 

This  arrangement  has  given  satisfactory  results. 

The  practical  difficulty  in  keeping  a  shield  correctly  on  a  curve  arises  from 
the  fact  that  however  carefully  each  ring  may  be  set  when  first  erected,  the 
repeated  pressure  of  the  rams  in  pushing  the  shield  forward  for  subsequent  rings 
tends  to  compress  the  tunnel  more  on  the  side  where  the  joints  are  wide  than  where 
they  are  tight,  and  so  to  flatten  the  curve  of  the  tunnel. 

The  amount  of  such  compression  will  vary  with  the  character  of  the  packing 
employed  in  the  circumferential  joints  of  the  tunnel  linings,  but  some  variation 
due  to  the  thrust  of  the  shield  always  takes  place. 

Another  result  of  the  continued  pressure  of  the  shield  rams  on  the  tunnel  lining 
is  that  the  bolts  in  the  circumferential  joints,  however  well  tightened  up  they  may 
be  when  a  ring  is  first  erected,  always  are  found  to  be  slack,  when  the  shield  has 
advanced  ten  or  fifteen  rings  forward. 

It  is  always  advisable  to  have  a  man  at  about  this  distance  behind  the  shield 
at  work  remedying  this  defect. 

The  rotation  of  the  shield  about  its  axis  is  a  common  circumstance,  and  is 
due  to  the  fact  that  one  or  more  of  the  hydraulic  rams  is  not  set  parallel  to  the  axis 
of  the  shield.  In  the  London  Clay  as  mentioned  above  this  movement  is  not  of  any 
great  importance,  but  in  some  other  types  of  shield  this  contingency  must  be  taken 
into  account.1 

The  grouting  round  the  tunnel  lining  must  never  be  permitted  to  get  in  arrear, 
and  the  operation  should  never  be  allowed  to  be  postponed  until  the  end  of  a  shift, 
or  until  two  or  three  rings  can  be  done  at  once. 

It  should  be  done  every  time  the  shield  is  moved  forward,  and  care  should 
be  taken  that  the  upper  portion  of  the  tunnel,  as  well  as  the  lower,  is  properly 
grouted. 

The  cost  of  a  Greathead  shield  for  a  tunnel  in  clay  of  from  11  feet  to  13  feet 
diameter  is  about  £450,  and  the  subsequent  cost  of  repair  and  keeping  in  working 

1  Proc.  Inst.  C.E.,  vol.  cl.  p.   17. 
IOI 


TUNNEL    SHIELDS 

order  is  comparatively  small  ;  the  main  outlay  being  on  the  hydraulic  rams  and 
pipe  connexions,  which  naturally  are  exposed  to  much  dirt  and  rough  usage. 

The  working  expenses  of  a  shield  in  London  Clay,  or  in  any  material  not 
requiring  compressed  air,  or  special,  precautions  in  the  front  of  the  shield  are  toler- 
ably uniform  so  far  as  labour  is  concerned.  The  supply  of  air  pressure  for  grouting 
the  tunnel  and  hydraulic  power  for  driving  the  shield  (when  this  is  not  supplied  by 
the  shield  gang  working  hand  pumps)  varies  of  course  with  the  local  conditions,  as 
does  the  first  cost  of  the  cast-iron  tunnel  lining.  In  general  the  cost  of  a  cast-iron 
tunnel  in  London  Clay  is  about  £2  5s.  to  £2  10s.  per  cubic  yard  of  excavation. 

The  working  gang  of  a  shield  12  feet  6  inches  in  diameter  or  thereabouts  is 
made  up  as  follows  1  : — 

1  ganger          .                                           ...  at  10s.  per  shift  of  10  hours 

4  miners          .  .  '        .           .           .  ,,     Qd.  per   hour 

4  miners'  labourers  .                                .           .  ,,     l^d.  ,, 

4  general  labourers  .           .           .           .  ,,      Id.    ,,         „ 

1  boy     .  ,,  4d.     „ 

These  men  carry  out  all  the  work  in  connexion  with  the  excavation,  the  driving 
of  the  shield,  the  erection  and  bolting  up  of  the  tunnel  lining,  and  grouting.  They 
also  haul  the  skips  containing  the  castings  from,  and  the  skips  laden  with  clay  to, 
the  nearest  "  turnout  "  of  the  contractors'  line,  generally  about  10  yards  in  the 
rear  of  the  shield. 

The  shield  work  is  usually  sublet  to  this  gang  at  prices  based  on  the  foregoing 
wages  list,  with  an  increasing  bonus  per  ring  above  a  certain  weekly  amount. 

In  ordinary  London  Clay  forty-five  rings  or  75  feet  of  tunnel  per  week  about 
12  feet  in  diameter  is  an  ordinary  rate  of  progress  when  the  excavation  is  done  by 
handwork. 

The  other  work  in  connexion  with  the  removal  of  the  spoil,  and  the  forwarding 
of  cast-iron  segments,  lime,  etc.,  to  the  shield  does  not  vary  much  from  similar 
work  in  ordinary  tunnels. 

In  tunnels  of  the  dimensions  of  the  one  under  consideration  it  is  the  usual 
practice  to  make  a  temporary  floor  of  the  excavated  clay  on  which  the  contractor's 
rails  are  laid,  the  clay  being  removed,  and  the  tunnel  cleaned  up  when  the 
permanent  way  is  put  in.  In  larger  tunnels  a  timber  floor  is  laid  down. 

It  is  found  that  up  to  300  yards  of  lead,  one  driver,  if  ponies  are  used,  and 
one  brakesman,  can  keep  pace  with  the  work  at  the  shield  in  tunnels  up  to  12  feet 
in  diameter.  When  the  shield  is  more  than  this  distance  from  the  shaft  or  outlet, 
a  double  service  is  required. 

Leaving  for  the  present  the  work  in  the  City  and  South  London  in  water- 
bearing material,  and  turning  to  the  consideration  of  special  features  in  shields 
employed  in  subsequent  works,  we  have  in  the  first  place  the  shields  partly  designed 
by  Mr.  Greathead  used  in  the  Glasgow  Subway,  which,  although  not  one  of  the 
London  Clay  tunnels,  may  be  considered  in  this  place. 

Glasgow  District  Railway 

This  work,  which  forms  a  circular  urban  line  in  Glasgow,2  was  commenced  in 
1892,  and  completed  in  1895.  Its  length  is  about  6,500  yards,  and  the  diameter 

1  These  are  Central  London  Railway  figures. 

2  P.roc-  !nst-  8^™™™  and  Shipbuilders  in  Scotland,  Jan.  28,  1896.     Simpson  on  "  Tun- 
nelling m  Soft  Material." 

1O2 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


of  the  tunnels  11  feet.     The  material  tunnelled  through  varied  from  brick  clay  to 
silty  sand,  work  in  the  latter  being  carried  out  by  timbering  in  front  of  the  shield. 


6  5 


inches  \J2 


PIT- 

179 


_  2  3  i£_fee£* 

FIG.  59.     THE  GLASGOW  DISTRICT  SUBWAY. 
The  Greathead  Shield.     Longitudinal  Section. 

The  type  of  shield  ussd   is   shown   in  Figs.  59  and  60,  and   naturally  closely 
resembles  the  City  and  South  London  shields. 

103 


TUNNEL    SHIELDS 

Each  shield  was  12  feet  1£  inches  in  diameter,  and  6  feet  6  inches  in  length. 
As  in  the  South  London  shield  the  skin  consisted  of  two  J  inch  plates  rivetted 
together,  the  diaphragm  or  bulkhead  was  similarly  placed  1  foot  only  from  the 
cutting  edge,  and  the  same  number  of  hydraulic  rams,  of  the  same  design  as  before, 
were  employed.  The  weight  of  the  shield  was  about  6  tons. 

A  feature  peculiar  to  this  shield  was  the  double  sliding  door  for  closing  the 
opening  in  the  diaphragm. 

The  pressure  pipes  are  made  to  run  vertically  and  horizontally,  so  as  to  leave 
the  door  in  the  diaphragm  clear  instead  of  forming  a  circle  as  in  the  City  and  South 
London  shield,  and  this  change  was  for  the  better  and  has  been  followed  since. 
The  engineer  of  the  line,  Mr.  Simpson,  has  put  on  record  his  opinion  that 
the  use  of  a  shield  in  iron-lined  tunnel  work  is  really  of  little  use,  and  further  says 
that  some  of  the  contractors  for  the  work  actually  removed  the  shields  from 
the  tunnel  as  being  hindrances  to  the  work,  and  broke  them  up. 

He  states  that  perhaps  the  use  of  the  shield  saved  a  little  timbering,  and  was 
useful  in  paving  off  smooth  the  circumference  of  the  excavation,  and  so  saved  a 
little  of  the  manual  work  in  excavation. 

The  above  observations  refer  to  work  in  the  brick  clay  before  mentioned, 
but  Mr.  Simpson,  as  will  be  seen  in  treating  of  the  "  assisted  shield,"  can  find  no 
advantage  in  a  shield  even  in  bad  material. 

As  for  the  shield  when  working  in  clay,  Mr.  Simpson  is  the  only  engineer 
having  practical  knowledge  of  the  work  who  has  formed  the  opinion  that  iron 
tunnels  are  better  built  without  one,  while  in  respect  to  its  work  in  open  ballast, 
it  is  true  that  the  Greathead  shield  was  made  for  work  in  clay,  and  that  it  requires 
certain  modifications  to  adapt  it  for  working  in  loose  materials.  It  is  clear,  however, 
from  the  paper  by  Mr.  Simpson,  from  which  the  above  criticisms  in  the  shield  are 
quoted,  that  the  slow  progress  made  on  the  Glasgow  Railway  was  in  part  due  to 
the  manner  of  conducting  the  work,  and  that  a  change  of  contractors,  when  the 
face  of  the  tunnel  was  under  the  river,  had  an  important  effect  on  the  rate  of 
advance,  the  same  shield  being  used  by  both  contractors. 

The  Central  London  Railway  Shields  1 

The  shields  used  on  this  railway  were  the  City  and  South  London  Railway 
shields  with  but  slight  modifications,  the  designs  for  them  having  been  laid  before 
Mr.  Greathead  before  his  death  in  1896.  The  ordinary  shield  used  in  the  section 
of  the  line  between  the  Marble  Arch  and  the  Post  Office  is  shown  in  Figs.  61  and  62, 
which  represents,  with  some  small  modifications  which  were  due  to  the  fact  that  the 
numerous  shields  used  on  the  railway  were  divided  among  various  manufacturers, 2 
the  type  of  shield  used  throughout  the  line. 2 

The  cylindrical  skin  is  shown  in  Fig.  62  as  being  made  of  two  J  inch  plates, 
but  in  some  of  the  later  shields  it  was  made  of  one  f  inch  plate,  generally  in  three 
pieces  with  a  covering  strip  at  the  joints. 

The  cutting  edge  with  the  planed  steel  adjustable  cutters  was  the  same,  save 
that  the  adjoining  segments  were  bolted  together,  as  that  of  the  City  and  South 
London  shield,  but  the  bulkhead  or  diaphragm  varied  from  the  original  model, 
^. 

1  Engineering,  Feb.   18  and  March  18,  1898;    and  Engineer,  Nov.  4,   11,  and  18     1898 
No  complete  record  of  this  railway  has  been  published. 

2  The  shield  figured  is  built  by  Markhams  of  Chesterfield. 

104 


I . .  i .  •  i  •  •  i  •  1 1  i  -|  \ 

inches/a  O  1  2  3 

FIG.  60.     THE  GLASGOW  DISTRICT  SUBWAY. 

The  Greathead  Shield.  Back  Elevation. 


105 


TUNNEL    SHIELDS 

in  that  the  door  was  larger,  being  6  feet  6  inches  high,  and  5  feet  wide.  The 
tendency  of  late  years  has  been  to  cut  away  more  and  more,  for  shields  working 
in  London  Clay,  the  diaphragm,  until  in  some  extreme  cases,  notably  in  shields  in- 
tended for  use  with  a  mechanical  excavator,  it  disappears  altogether,  and  the  shield 
appears  simply  as  a  cylinder  of  steel  plate,  sufficiently  stiffened  by  internal  castings 
to  admit  of  propulsion  by  hydraulic  rams. 


FIG.  61.     THE  CENTEAL  LONDON  RAILWAY. 
The  Greathead  Shield.     Back  Elevation. 

But  in  the  ordinary  Central  London  Railway  shield  this  tendency  was  kept 
within  limits,  and  as  a  result  the  shields,  as  a  general  rule,  kept  fairly  rigid,  although 
all  to  a  more  or  less  marked  degree  widened  in  the  horizontal  diameter  with  a  cor- 
responding flattening  in  the  vertical  height. 

The  castings  carrying  the  rams  and  the  rams  themselves  are  the  same  as  on 

106 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

the  earlier  shields,  the  supply  pipes  being  arranged  to  follow  the  shape  of  the  door 
in  the  diaphragm  in  order  to  avoid  obstructing  the  work  of  the  shield. 


In  some  of  the  later  shields  employed  on  the  railway,  the  arrangement  of  hand 
pumps,  with  differential  valves  for  working  at  high  and  low  pressure,  was  aban- 
doned in  favour  of  a  neat  air  engine,  similar  to  that  shown  in  the  shield  in  Fig. 

107 


TUNNEL    SHIELDS 

69,  *  which,  taking  water  either  from  the  tanks  on  the  shield  or  from  the  pressure 
supplied  by  the  London  Hydraulic  Company,  in  the  one  case  compressed  the 
water,  in  the  other  intensified  the  pressure  in  the  mains,  so  as  to  supply  pressure 
to  the  shield  rams  of  the  power  required. 

These  air  engines  were  fixed  on  a  shelf  on  the  diaphragm  immediately  above 
the  doorway.     Their  use  has  the  advantage  that  during  the  pushing  forward  of  the 


FIG.  63.     CENTRAL  LONDON  RAILWAY. 

The  Greathead  Shield,  adapted  for  use  with  Mechanical  Excavator.     Half  Back  Elevation,  and  Half 

Front  Elevation. 

shield  the  shield  gang  are  set  free  to  prepare  for  the  erection  of  the  web  ring  of 
tunnel  lining.  The  supply  of  air  for  the  engine  is  easy,  as  air  at  a  pressure  of 
80-100  pounds  per  square  inch  is  required  in  any  case  at  the  shield  for  the  working 
of  the  grouting  pan.  A  minor  advantage  in  the  use  of  these  air  engines  is  that 
their  use  does  away  with  the  hand  pumps  which  take  up  room  in  the  shield,  while 
the  air  engine,  placed  as  it  is  above  the  door,  is  entirely  out  of  the  way  of  the  work- 
men. 

1  The  machine  figured  is  one  of  Hayward  Tylers  manufacture. 

108 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


The  shield  shown  in  Figs.  61  and  62  is  the  type  usually  employed  between 
1895  and  1903  for  the  tunnels  of  from  11  feet  8  inches  to  13  feet  diameter  in  London 
Clay,  but  a  modified  shield  made  for  use  with  a  patent  excavating  machine  x  was 
also  employed,  and  this  presents  one  feature  of  interest  (see  Figs.  63  and  64).  The 
whole  of  the  diaphragm,  which  is  an  essential  part  of  the  shield  as  imagined  by 
Barlow  and  Greathead,  has  here  disappeared,  nothing  being  left  of  it  except  a 
circular  flitch,  formed  of  two  f  inch  plates,  A  A,  Fig.  64,  between  the  cast-iron 
segments  of  the  cutting  edge  and 
those  carrying  the  rams.  The 
shield  is,  in  fact,  merely  a  portable 
frame  which  holds  up  the  roof  of 
the  excavations  while  the  iron 
lining  of  the  tunnel  is  being  put 
in,  but  which  affords  no  protec- 
tion whatever  against  any  coming 
in  of  the  face.  The  plates  A,  A, 
about  1  foot  4  inches  deep,  which 
are  all  that  is .  left  of  the  dia- 
phragm, do  little  more  towards 
stiffening  the  shield  than  acting 
as  cover-plates  over  the  joints  in 
the  castings  before  and  behind 
them.  Round  their  inner  edge  is 
rivetted  a  channel  iron  B,  B,  which 
stiffens  them  in  some  measure, 
and  serves  as  a  frame  in  which 
planks  can  be  placed  for  closing 
the  face  when  necessary. 

At  the  lower  part  of  the 
opening  of  the  shield  an  apron 
C,  C,  is  fixed,  the  object  of  which 
is  to  keep  the  clay  broken  down 
by  the  excavator  from  filling  up 
the  bottom  of  the  shield. 

The  excavator  itself  is  shown 
in  Fig.  65  (in  which  the  shield 
figured  varies  a  little  from  that 

just  described,    the  apron  being 

a    ,,  j   j-i         i  i    •  J.-P  FIG.  64.     CENTEAL  LONDON  RAILWAY. 

natter  and  the  channel  iron  stif- 

c  .-.       ,  -.      .,        „  ,,       ,        The  Greathead  Shield,  adapted  for  use  with  Mechanical 

fener  on  the  tunnel  Side  of  the  f  Excavator.     Longitudinal  Section. 

plate).  This  machine  is  the  in- 
vention of  Mr.  T.  Thomson,  and  when  tried  in  1897  on  the  Central  London  Rail- 
way gave  fairly  satisfactory  results,  but  was  only  used  in  one  tunnel,  the  frequent 
failure  of  the  electrical  motor  and  connexions  causing  the  loss  of  so  much  time  that 
the  increased  rate  of  excavation  obtained  when  the  machine  was  working  was 
made  of  no  effect. 

In  a  larger  tunnel  than  one  of  1 1  feet  8  inches  diameter,  and  with  more  atten- 
tion paid  to  the  motor  and  electrical  equipment,  this  machine  would  be  worth 

1  Engineer,  Nov.   18,   1898. 
109 


TUNNEL    SHIELDS 

further  trial,  in  view  of  the  fact  that  with  it  the  number  of  men  employed  can  be 
reduced  by  nearly  one-half. 

The  excavator  consists  of  a  carriage  or  frame  on  wheels,  carrying  on  it  a  top 


platform  having  a  certain  freedom  of  revolution  in  a  horizontal  direction.  This  top 
platform  carries  an  electric  motor  A,  which  actuates  the  various  motions  of  the 
^machine. 


no 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

In  the  front  of  the  platform  is  a  shaft  B,  worked  directly  by  the  gearing  shown 
between  it  and  the  motor.  On  this  shaft  is  placed  loosely  an  arm  C,  on  which 
is  fixed  a  ladder  of  buckets  D,  D,  D,  similar  to  that  of  a  dredging  machine. 

This  ladder  is  made  to  travel  by  the  revolution  of  the  shaft  B.  The  arm  C  is 
raised  or  depressed  by  throwing  into  gear  the  wheel  D,  which  winds  or  unwinds  a 
wire  rope  passing  over  a  pulley  between  two  cantilevers  or  frames  E,  E. 

To  operate  the  machine  the  carriage  is  brought  forward,  so  that  the  arm  C 
carrying  the  excavating  buckets  is  in  the  position  shown  in  the  figure,  by 
means  of  the  drums  F,  F  (which  can  be  geared  at  will  with  the  motor  A),  on  which 
are  wound  wire  ropes  fastened  to  the  lining  of  the  tunnel  already  built.  The  shaft 
B,  carrying  with  it  the  ladder  of  buckets,  is  made  to  revolve  at  the  same  time  that 
the  lifting  barrel  D,  which  raises  the  arm,  and  makes  it  with  the  buckets  go  through 
the  same  operation  in  a  vertical  plane,  as  a  dredge  does  in  the  horizontal  bed  of  a 
river,  is  put  in  operation. 

When  one  upward  cut  is  complete  the  arm  C  is  lowered  again,  and  then  by 
means  of  the  slewing  barrel  at  the  rear  of  the  machine  moved  to  one  side  to  take 
another  cut,  and  then  again  raised  with  the  bucket  ladder  working.  As  the 
bucket  ladder  revolves,  the  clay  brought  up  by  each  bucket  is,  on  the  bucket 
turning  over  the  shaft  B,  dropped  into  a  truck  (not  shown  on  the  drawing),  the 
frame  of  the  excavator  being  made  large  enough  to  allow  of  a  truck  passing 
under  it. 

The  excavator,  as  can  be  seen  from  the  figure,  can  reach  all  the  face  of  the  clay, 
save  for  a  few  inches  round  the  circumference  of  the  shield.  This  remaining 
material,  however,  was  easily  cut  away  by  the  shield  as  it  advanced. 

When  the  excavator  has  cut  out  20  inches  of  clay  or  thereabouts,  and  the 
shield  is  pushed  forward  far  enough  to  admit  of  the  erection  of  a  ring  of  cast-iron 
lining,  it  is  run  back  some  10  feet  to  enable  the  miners  to  proceed  with  the  erection 
of  the  ring,  which  being  done,  the  excavator  is  set  to  work  again,  and  the  cycle 
of  operations  repeated. 

The  rate  of  progress  with  this  machine  was  about  double  that  attained  by 
ordinary  mining  in  front  of  the  shield. 

As  a  general  rule,  mechanical  excavators  are  objectionable  in  small  tunnels, 
for  the  reason  that  they  form  a  serious  obstruction  in  the  working  face,  and  that, 
should  they  break  down,  they  are  likely  to  entirely  stop  work  for  some  time  until  they 
can  be  repaired  or  removed. 

This  fault  the  Thomson  excavator  has  not  ;  in  case  of  a  breakdown  it  can  be 
pushed  back  from  the  face,  and  work  in  the  shield  continued  in  the  ordinary  way, 
the  ships  or  trucks  for  removing  the  clay  passing  through  the  frame  of  the  excavator 
without  difficulty,  the  working  platform  of  the  machine  being  made  high  enough 
to  allow  of  this. 

The  motor  used  in  driving  this  machine  on  the  Central  London  Railway  was  a 
100  ampere  motor  at  200  volts. 

But  in  small  tunnels  of  11  to  13  feet  in  diameter  a  machine  of  this  type  is  very 
expensive  to  work,  from  the  fact  that,  owing  to  limitations  of  space,  and  particularly 
of  headroom,  many  of  the  operations  of  the  machine  are  carried  out  at  a  mechanical 
disadvantage.  For  instance,  the  lifting  cable  of  the  arm  carrying  the  buckets, 
which  passes  over  the  pulley  between  the  cantilevers  E,  E,  is  obviously  attached 
much  too  near  the  fulcrum  of  the  arm  to  lift  this  latter  save  at  an  expenditure  of 
power  compared  with  the  actual  pressure  required  to  cut  the  clay  of  some  three  to 

in 


one  ; 
arm. 


TUNNEL    SHIELDS 

and  this  cannot  be  improved  without  limiting  the  vertical  movement  of  the 


This,  as  said  above,  might  be  obviated  in  a  larger  tunnel,  but  up  to  the  present 
this  type  of  excavator  has  not  been  tried  a  second  time. 

Another  mechanical  excavator  was  tried  in  1897,  also  on  the  Central  London 
Railway,  of  a  different  character.  With  this  machine,  designed  by  Mr.  Price,  the 
contractor  for  the  section  of  the  line  from  Shepherd's  Bush  to  the  Marble  Arch, 
a  shield  of  the  Greathead  type,  but  entirely  divested  of  any  diaphragm,  was  used 


FIG.  66.     CHARING  CROSS  AND  HAMPSTEAD  RAILWAY,  LONDON. 
Price's  Combined  Shield  and  Mechanical  Excavator.     Back  Elevation. 

(like  the  first  described  as  employed  with  Thomson's  excavator,  but  without  the 
apron,  which  with  Price's  machine  was  not  required). 

The  machine  itself  consisted  of  a  number  of  arms  carrying  cutting  chisels, 
which  (the  arms)  were  radial  to  a  shaft  occupying  the  axis  of  the  tunnel  to  which 
the  frame  carrying  it  was  attached.  This  shaft  was  rotated  by  electrical  power, 
and,  the  arms  revolving  with  it,  the  clay  face  was  removed  by  the  chisels. 

The  main  defect  in  the  design  was  that,  when  working  on  a  curve,  there  was 
too  much  spring  in  the  long  axle  to  allow  the  machine  to  excavate  more  on  one  side 

112 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

than  on  the  other,  and  the  independence  of  the  excavator  from  the  shield  accen- 
tuated this  defect. 

The  driving  power  also  was  applied  in  the  wrong  place,  namely  at  the  axle 
instead  of  at  the  circumference  of  the  machine. 

These  defects  have  been  remedied  by  Mr.  Price  in  the  excavator  shown  in 
Figs.  66  and  67,  which  up  to  the  present  must  be  regarded  as  the  best  mining  imple- 
ment for  London  Clay  work  invented. 

Charing  Cross  and  Hampstead  Shield  and  Excavator 

It  has  been  in  regular  use  on  the  Charing  Cross  and  Hampstead  Railway  for 
some  time,  and  has  attained  in  favourable  circumstances  the  extraordinary  speed  of 
108  rings  of  tunnel  lining,  11  feet  8  inches  diameter,  or  180  feet  per  week,  and 
this  without  many  intervals  of  idleness  due  to  a  breakdown  of  the  machinery.1  ?  ! 

It  consists  of  a  combination,  in  one  frame,  of  the  shield  and  excavator,  the 
latter  being  mounted  on  girders  within  the  former,  and  both  moving  forward  to- 
gether. 

The  excavator  is  of  the  rotary  type,  and  the  clay  cut  from  the  face  is,  as  it 
falls  into  the  invert  of  the  shield,  picked  up  by  buckets  which  revolve  with  the 
arms  of  the  excavator,  and  discharge  in  turn  as  each  reaches  the  soffit  of  the  shield, 
the  clay  so  gathered,  by  means  of  shoots  into  trucks,  or  as  in  the  machine  used 
by  Mr.  Price,  into  a  mechanical  conveyor.  Many  patents  of  this  character  are  to 
be  found  in  the  Patent  Office  records,  and  some  of  them  are  included  in  the  list  of 
tunnel  patents  given  in  this  volume.  The  two  Price  patents  are,  however,  the  only 
machines  of  a  rotary  type  which  have  been  put  to  the  test  of  actual  work. 

In  detail  the  machine  is  described  below. 

In  an  ordinary  Greathead  shield,  which  differs  only  from  the  Central  London 
Railway  type  in  the  arrangement  and  number  of  the  hydraulic  rams,  which  are 
unequally  spaced,  six  being  in  the  lower  part  of  the  shield,  and  four  in  the  upper, 
and  in  the  absence  of  a  diaphragm,  save  in  the  rudimentary  form  of  two  f  inch 
plates  which  form  a  ring  round  the  shield  hardly  wider  than  the  flanges  of  the 
cutting  edge,  and  of  the  ram  castings,  are  fixed  two  horizontal  girders  A,  A, 
2  feet  9  inches  apart  and  each  formed  of  two  channel  irons  9  inches  by  3  inches  by 
f  inch,  with  flanges  f  inch  thick.  These  are  placed  8  inches  above  the  horizontal 
diameter  of  the  shield,  and  are  bolted  at  the  ends  to  the  ram  castings  which  are 
made  of  special  pattern  to  receive  them.  They  thus  serve,  in  addition  to  their  use 
in  carrying  the  excavating  machinery,  as  very  effective  tie  bars  for  keeping  the 
shield  in  shape. 

At  the  centres  of  these  girders  and  beneath  them,  so  that  the  axis  of  the  shaft 
coincides  with  the  longitudinal  axis  of  the  shield,  are  fixed  two  journals  B,D, 
carrying  the  main  shaft  C  of  the  excavator,  which  is  8  inches  in  diameter. 

The  rear  journal  D  is  constructed  to  serve  also  as  a  bearing  block,  and  against 
it  the  shaft  can  be  adjusted  by  nuts  accessible  from  the  back  of  the  shield. 

In  front,  and  bearing  against  a  collar  E  on  the  shaft  C,  is  fixed  a  cast  metal 
hub  F,  to  which  are  bolted  six  radial  arms  G,  G,  G.  Each  of  these  arms  or  spokes 
is  formed  of  two  channel  irons  6  inches  by  3  inches,  which  are  braced  together  by 
other  channel  bars  serving  also  as  holders  for  the  removable  cutters  or  chisels 
K,  K,  K. 

1  360  rings  have  been  erected  in  four  consecutive  weeks. 

113  I 


TUNNEL    SHIELDS 


These  chisels  are  attached  to  the  channel  bars  in  which  they  lie,  by  plates 
and  bolts,  very  much  in  the  manner  of  a  chisel  in  an  ordinary  planing  machine. 

They  are  arranged  at  varying  distances  from  the  axis  of  the  central  shaft 
on  the  different  arms,  so  that  when  the  arms  are  set  in  motion,  the  whole  surface 
of  the  clay  face  is  operated  on  in  each  complete  revolution  of  the  shaft. 

The  six  arms  are  held  together  by  circumferential  plates  H,  H,  which  are  bolted 
to  them,  and  not  only  greatly  increase  their  strength,  but  also  serve  as  the  base 
plates  to  which  are  attached  the  circular  rack  L,  by  which  the  arms  are  driven  round, 

and  also  the  buckets  J,  J, 
T  whose  function  is  to  pick 
up  the  debris  thrown  down 
by  the  cutters,  and  dis- 
charge it  clear  of  the 
shield. 

It  will  be  seen,  there- 
fore, that  the  excavator  is, 
in  principle,  a  large  revol- 
ving wheel,  having  fixed 
to  its  spokes  cutters  or 
chisels  which  slice  away 
the  clay  face  ;  the  wheel 
being  mounted  on  a  shaft 
concentric  with  the  shield 
to  which  it  is  secured. 

This  wheel  is  made  to 
revolve  by  a  chain  of 
wheels  M ,  N;0,P;  Q,R; 
S,  L ;  M  being  on  the 
axis  of  the  electric  motor 
T,  and  L  the  circular  rack 
of  the  excavating  wheel. 
The  chain  of  wheels  is  sup- 
ported on  castings  resting 
on  or  under  the  girders 
A,  A,  and  the  motor  T  is 
fixed  in  a  frame  carried  in 
part  by  the  ram  casting  of 
the  shield,  and  in  part  by 
a  channel  iron  between  A 
and  the  invert  of  the 
shield.  The  motor  is  of 

60  H.P.,  and  runs  at  about  500  revolutions  per  minute,  the  usual  speed  of  the 
excavating  wheel  being  about  one  and  one-half  revolutions  per  minute. 

In  actual  work  the  wheels  M  and  N  are  encased  in  a  thin  metal  shield,  and 
the  remaining  gearing  is  partly  protected  against  dirt.  Round  the  lower  half  of 
the  shield  is  fixed  an  apron  plate  7,  which  keeps  the  clay  from  choking  the  teeth 
of  the  rack  L,  this  plate  being  secured  to  the  shield  by  bent  bars  rivetted  to  the 
|  inch  plates  of  the  diaphragm. 

In  addition  to  the  chisels  K,  K,  K,  the  later  shields  of  Mr.  Price's  patterns  have 

114 


FIG.  67.     CHARING  CROSS  AND  HAMPSTEAD  RAILWAY. 
Price's  Combined  Shield  and  Mechanical  Excavator.     Longitudinal 
Section. 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

fixed  on  the  central  hub,  F,  but  placed  eccentrically  to  its  centre,  a  kind  of  toothed 
scraper  V,  which  is  intended  to  remove  the  centre  of  the  face  and  also  to  keep  the 
excavator  steady  in  line. 

A  further  improvement,  which  in  actual  work  has  answered  very  well,  devised 
to  meet  the  difficulty  of  excavating  more  on  one  side  than  on  the  other  when  the 
shield  is  to  be  driven  round  a  curve,  has  been  suggested  by  Messrs.  Markham,  the 
constructors  of  these  machines.  It  consists  in  fixing  at  the  ends  of  the  arms  G,  a 
cutter  which  can  by  cams  or  eccentric  arrangement  be  projected  beyond  the  end 
of  the  arm  and  beyond  the  skin  of  the  shield  for  a  certain  length  of  the  circum- 
ference of  the  face,  and  so  for  that  distance  making  a  wider  excavation  into  which 
the  shield  can  be  turned. 

In  addition  to  the  mechanism  shown  in  the  figures  there  is  fixed  to  the  girders 
A,  A,  a  large  shoot  (indicated  in  outline  by  double  dotted  lines)  which  receives  the 
clay  discharged  from  the  buckets  J,  J,  and  delivers  it  to  a  conveyor,  also  worked 
by  electricity,  and  somewhat  of  the  character  of  the  one  shown  in  Figs.  188  and 
189,  but  simpler,  by  which  the  clay  is  carried  some  30  feet  from  the  shield  and 
discharged  into  skips. 

The  same  frame  or  carriage  which  supports  the  conveyor  carries  also  a  small 
compressor  for  the  hydraulic  rams  of  the  shield  and  the  starting  switches  for  con- 
trolling the  mechanism  of  the    conveyor  motor  and  the  excavator  motor.     This 
frame  or  carriage  runs  on  rails  temporarily  fixed  on  either  side  of  the  tunnel. 
The  method  of  working  one  of  these  shields  is  as  follows  : — 
When  the  erection  of  a  tunnel  ring,  during  which  operation  the  shield  is  per- 
force at  rest,  is  completed,  the  shield  is  put  in  motion  by  starting  the  hydraulic 
rams,  at  the  same  time  that  the  excavator  is  made  to  revolve  by  starting  the  motor 
T.     The  full  control  of  all  the  machinery  is  in  the  hands  of  an  operator  seated  on 
the  carriage  of  the  conveyor  who  has  within  his  reach  the  rheostats  for  starting 
the  motors  of  the  excavator  and  conveyor,  and  the  valves  of  the  hydraulic  gear. 
As  the  cutters  K,  K,  on  the  arms  G,  G,  break  off  the  clay  in  the  face,  the  buckets 
J,  J,  scoop  it  up,  as  they  revolve,  from  the  invert  of  the  excavation,  carry  it  round, 
and  as  one  after  another  they  reach  the  top,  discharge  it  downwards  into  the  main 
shoot  (indicated  in  double  dotted  lines),  whence  the  conveyor  receives  it. 

The  satisfactory  working  of  the  shield  depends  on  the  correct  adjustment  to 
each  other  of  the  two  motions,  the  advance  forward  of  the  shield,  and  the  revolving 
action  of  the  excavator,  and  this  requires  very  careful  watching  on  the  part  of  the 
man  controlling  the  motors  and  rams. 

As  soon  as  he  perceives  by  means  of  an  ammeter  that  the  resistance  of  the 
excavating  machine  is  increasing,  due  to  the  fact  that  the  shield  is  going  forward 
more  quickly  than  the  excavator  can  remove  the  clay,  he  reduces  the  pressure  in 
the  rams  ;  if,  on  the  other  hand,  he  finds  that  the  excavator  is  making  light  cuts,  he 
increases  the  pressure  in  the  hydraulic  rams  to  keep  the  shield  hard  up  to  the  clay 
face  as  it  is  excavated. 

As  said  above,  this  machine  has  given  very  satisfactory  results,  and  in  material 
like  London  Clay  is  undoubtedly  a  very  excellent  tool.  It  is  not,  of  course,  suited 
for  any  material  in  which  stones  or  hard  rock  is  likely  to  be  encountered  as,  even 
if  the  nature  of  the  material  in  the  face  were  discovered  in  time  to  prevent  injury 
to  the  excavating  tools,  there  is  no  possible  means  of  getting  to  the  face  of  the  shield 
to  work  the  excavation  by  hand  except  by  removing  the  whole  excavator  from  the 
shield. 


TUNNEL    SHIELDS 


The  risk  of  meeting  hard  material  in  the  London  Clay  is,  of  course,  slight,  except 
at  the  junction  of  the  London  Clay  proper  with  the  hard  red  clay  of  the  Reading 
beds  beneath,  where  bands  of  hard  rock  are  sometimes  found. 

The  manner  in  which  the  feed  of  the  excavator  is  regulated  by  the  advance 
of  the  shield  is  very  sound,  not  merely  because  the  feed  is  efficient,  but  because 
by  making  the  shield  advance  with  the  excavation  there  is  never,  as  in  ordinary 
miners'  work,  in  front  of  a  shield  an  area  of  face  and  circumference  unprotected 
and  unsustained. 

The  shield  indeed,  when  the  machine  is  working  well,  is  never  at  rest,  except 
during  the  period — twenty  minutes — necessary  to  erect  each  ring  of  tunnel  cast- 
ings, which  is  of  itself  a  recommendation. 

The  men  necessary  to  work  the  shield  are  : — 

1  ganger, 

2  miners, 

6  labourers, 
1  boy, 

or  nine  men  and  a  boy  as  compared  with  thirteen  men  and  a  boy  for  the  ordinary 

Greathead  shield  when  mining  by  hand. 

This  gang  working  ten  and  one- 
half  shifts  of  ten  hours  each  per  week 
has  made  a  maximum  advance  of  180 
feet  per  week,  or  over  17  feet  per  shift  : 
a  great  improvement  on  any  previous 
rate  of  progress. 

The  principal  practical  difficulty  at 
present  in  the  working  of  this  machine 
is  the  tendency  of  the  shield  to  get  out 
of  line,  unless  very  carefully  watched 
when  in  movement. 

The  Great  Northern  and  Strand 
Railway 

Before  leaving  the  subject  of 
Greathead  shields  as  used  in  London 
Clay  in  iron  tunnels  of  from  10  to  14 
feet  diameter,  attention  may  be  called 
to  one  of  the  latest  patterns  of  shield 
built  for  Messrs.  Walter  Scott  and  Com- 
pany, the  contractors  for  the  Great 
Northern  and  Strand  Railway,  who 
were  also  contractors  for  Mr.  Great- 
head's  original  City  and  South  London 
Railway.  This  shield,  Figs.  68  and  69, 
though  all  its  features  had  been  adopted 
in  one  tunnel  or  another  previously,  may  be  usefully  compared  with  the  City  and 
South  London  prototype  of  eighteen  years  before  ;  it  is  interesting  both  in  its 
resemblances  and  in  its  differences. 

The  skin,  instead  of  being  composed  of  two  plates  £  inch  thick,  consists  of  one 

116 


FIG.  68.     GREAT  NORTHERN  AND  STRAND  RAIL- 
WAY, LONDON. 
Greathead  Shield.     Longitudinal  Section. 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


£  inch  plate  in  three  sections,  with  cover-plates  outside  the  joints.  The  cutting 
edge  is  unaltered,  and  the  cutting  knives  also. 

The  diaphragm,  as  in  the  shields  for  use  with  excavating  machines,  has  almost 
disappeared  ;  and  the  shield  is  without  protection  from  any  collapse  of  the  face, 
or  irruption  of  water  :  experience  having  shown  that  there  is  so  little  risk  of  either 
accident  in  London  Clay  that,  given  reasonable  care  in  sinking  trial  borings  before- 
hand, their  possibility  may  be  neglected. 

The  cast-iron  ring,  formed  of  segments,  which  stiffens  the  shield  is  unaltered, 
but  in  the  arrangement  of  the  rams  themselves  there  are  considerable  alterations. 

The  valves  controlling  the  supply  of  water  to  the  rams  are  concentrated  in 
one  cluster  at  the  side  of  the  tunnel,  instead  of  being  distributed,  two  to  each  ram, 


FIG.   69. 


GREAT  NORTHERN  AND  STRAND  RAILWAY,  LONDON. 
Greathead  Shield.     Back  Elevation. 


around  the  shield,  and  can  be  controlled  by  one  man  who  can  reach  every  valve 
without  interfering  with  the  other  work  of  the  shield. 

The  hydraulic  pressure  is  obtained  by  a  small  air-driven  compressor  at  the 
top  of  the  shield  instead  of  by  manual  labour  ;  and  the  rams  themselves  have  been 
increased  in  number  (the  Great  Northern  and  Strand  tunnel  is  only  about  1  foot 
6  inches  larger  in  diameter  than  the  earliest  City  and  South  London  one)  from  six 
to  eight,  and  instead  of  being  symmetrically  arranged  in  the  shield  they  are  dis- 
posed, as  in  the  Price  shield  on  the  Charing  Cross  and  Hampstead  Railway,  to  exert 

117 


TUNNEL    SHIELDS 

the  greatest  pressure  below  the  horizontal  axis  of  the  shield,  five  being  in  the  lower 
half  and  only  three  in  the  upper. 

The  increase  in  the  number  of  rams  is  an  improvement  :  the  more  rams  there  are 
the  more  easy  is  the  keeping  of  the  shield  in  good  alignment,  and  the  less  chance 
there  is  that  a  breakdown  in  one  of  them,  if  it  occurs  during  the  absence  of  the 
engineer  in  charge,  may  seriously  affect  the  direction  of  the  shield  ;  but  in  a  solid 
material  like  London  Clay  the  massing  of  the  rams  in  the  lower  half  of  the  shield 
hardly  appears  of  much  advantage.  In  loose  material,  where  the  shield  has  a  ten- 
dency to  sink,  the  arrangement  is  advantageous. 

The  Greathead  Shield  in  London  Clay  for  Tunnels  of  Larger  Diameter 

The  shields  so  far  considered  are  those  employed  in  tunnels  of  about  14  feet 
diameter  and  under,  which  present  a  face  of  clay  sufficiently  small  to  be  easily 
worked  in  the  manner  already  described,  without  the  use  of  scaffolding. 

But  in  the  Waterloo  and  City  Railway  l  and  the  Central  London  Railway, 
both  commenced  under  the  supervision  of  Mr.  Greathead,  station  tunnels  of  23  and 
21  feet  diameter  respectively  were  built  with  shields,  and  since  that  time  tunnels  of 
30  feet  diameter  have  been  constructed  in  London  Clay.  Since  then  the  Great 
Northern  and  City  Railway  has  been  built  with  tunnels  throughout  its  entire 
length  of  16  feet  diameter,  and  tramway  tunnels  with  a  diameter  of  17  feet  are 
now  being  constructed  under  Holborn.  These  larger  shields  require  considerable 
alterations  from  the  type  successfully  operated  in  constructing  smaller  tunnels,  and 
in  some  respects  bear  more  resemblance  to  the  Beach  shield  than  to  the  Greathead 
one. 

Their  size  compels  the  use  of  transverse  platforms  on  which  the  miners  may 
stand  to  work,  and  the  employment  of  mechanical  erectors  for  placing  the  cast- 
iron  segments  of  the  tunnel  lining  in  position  ;  and  in  addition  the  large  face  of 
clay  exposed  in  front  requires  support  as  it  is  excavated  in  advance  of  the  shield. 
The  first  two  requirements  caused  the  disappearance  of  the  solid  diaphragm  with 
a  central  door,  which  was  a  distinctive  feature  of  both  the  Barlow  and  Greathead 
shields,  and  the  third  required  the  provision  of  rams  in  the  front  of  the  shield,  to 
serve  the  same  purpose  as  stretchers  in  an  ordinary  timbered  tunnel  face. 

The   Central   London   Railway  and  Waterloo   and   City   Railway  Station 

Shields 

The  shield  devised  to  meet  these  special  requirements,  and  used  on  the  Central 
London,  and  Waterloo  and  City  Railways  is  shown  in  Figs.  70,  71  and  72.  The 
shields  in  the  two  undertakings  are  alike,  and  differ  only  in  diameter,  those  of 
the  former  railway  being  21,  those  of  the  latter  23  feet  in  internal  diameter. 

The  skin  of  the  shield,  which  in  length  over  all  is  6  feet  10  inches,  is  composed 
of  two  |  inch  plates,  lap  jointed  with  countersunk  rivets.  The  cutting  edge  A, 
which  is,  unlike  the  smaller  shields,  not  provided  with  cutting  knives,  consists  of 
twenty-two  segments  as  does  the  cast-iron  ring  B  behind,  carrying  the  rams,  also 
twenty-two  in  number. 

1  Proc.  Inst.  C.E.,  vol.  cxxxix.  p.  50.  This  was  the  first  tube  railway  in  London  where 
the  station  tunnels  were  made  with  cast-iron  linings ;  on  the  City  and  South  London  Railway 
(first  contract)  brick  tunnels  were  built  for  the  stations. 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

Between  these  two  cast-iron  rings  is  a  thick  steel  plate  diaphragm  C,  consisting 
of  eleven  segments,  the  joints  of  which  break  joint  with  those  of  the  cast-iron  rings. 


.HctLf  Bci£Jr  Elevation      JtciLf  CrogS  Section  . 


£        }          J         1         ^         J         e  \io  feet. 

FIG.  70.  _  THE  CENTRAL  LONDON  AND  WATERLOO  AND  CITY  RAILWAY. 
Shield  for  Tunnels  of  21  and  23  feet  internal  diameter. 

This  diaphragm,  as  is  seen  in  Fig.  70,  is  little  more  than  a  cover-plate  to  the  joints 
of  the  cast-iron  segments. 

119 


TUNNEL    SHIELDS 

The  arrangement  of  the  pressure  pipes  to  the  rams  is  shown  in  the  Figures, 
70  and  71,  but  the  tanks  from  which  the  water  is  drawn  and  to  which  the  waste 

is  returned  are  not  shown,  and 
generally  are  not  carried  on  the 
shield  itself,  but  on  a  gantry  behind 
it. 

The  bracing  of  a  shield  of  such 
large  diameter  is  formed  by  two 
vertical  girders  D,  D,  and  three 
horizontal  girders,  E,  Ev  E2,  or 
tables  composed  of  steel  plates 
stiffened  with  channel  irons.  These 
girders  are  secured  at  their  ends 
to  the  ram  castings,  and  to  each 
other  at  their  intersections  by  heavy 
angle  irons.  The  lowest  of  the 
three  horizontal  girders  or  tables, 
E2,  has  the  upper  surface  planked 
over,  and  it  is  at  this  level  that  the 
advance  heading,  when  one  is 
driven,  is  made.  The  rectangular 
framing  formed  by  these  five  frames 
make  the  shield  extremely  rigid. 
The  two  upper  tables  E,  EI;  are 
made  so  that  the  channel  irons  en- 
close hydraulic  rams  F,  F,  F,  the 
heads  of  which  G,  O,  are  composed 
of  angle  irons  and  a  plate,  which  are 
secured  to  a  sliding  table  H,  on 
which  the  miners  work.  As  the 
excavation  in  front  of  the  shield 
progresses,  the  rams  are  forced  for- 
ward, and  support  the  face. 

The  hydraulic  pressure  for 
these  "  face  "  rams,  as  they  are 
called,  is  arranged  in  the  same  way 
as  that  which  operates  the  ordinary 
shield  rams. 

Behind  the  vertical  girders 
D,  Z>,  are  fixed  two  hydraulic  erec- 
tors, for  fixing  in  their  places  the 
segments  of  the  tunnel  lining.  Each 
erector  consists  of  an  hydraulic 
cylinder  J,  the  piston  of  which 

terminates  in  a  fork  K,  to  which  the  tunnel  segments  can  be  secured.  The 
piston  can  be  extended  or  withdrawn  by  passing  the  pressure  through  the 
pressure  pipe  L,  or  M,  as  required.  The  pressure  is  regulated  in  this,  as  in  all 
other  movements  of  the  erector,  by  valves  placed  near  one  of  the  floors  of  the 
shield. 


FIG.  71.     THE  CENTRAL  LONDON  AND  WATERLOO  AND 

CITY  RAILWAYS. 
Shield  for  Tunnels  of  21  and  23  feet  internal  diameter. 


1 2O 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


The  cylinder  J  is  pivotted  in  an  axis  O,  which  also  carries  a  drum  P,  round 
which  is  passed  a  chain,  the  ends  of  which  are  attached  to  the  pistons  of  two  verti- 
cal cylinders  R,  R.  By  manipulating  these  cylinders,  the  drum  P  can  be  made 
to  revolve,  and  with  it  the  cylinder  J. 

The  working  of  the  shield  is  similar  to  that  of  the  smaller  one,  except 
that  the  use  of  piles  or  wedges  to  break  down  the  clay  is  not  so  effective 
on  the  large  shield  as  in  the  smaller,  even  when  a  heading  is  used,  which  is  not 
always  the  case.  The  clay  is  excavated  from  the  top  downwards,  and  held  up 
until  the  shield  is  ready  to  move  by  a  few  "  soldiers  "  wedged  against  the 
framing  of  the  shield  and  the  face  rams,  which  are  advanced  as  the  excavation 
proceeds. 

When  the  shield  is  advancing,  the  wedges,  if  any,  are  knocked  out,  and  the 
whole  pressure  of  the  face  is  taken  by  the  face  rams.  The  ordinary  shield  rams, 
in  driving  forward  the  shield,  overcome 
the  backward  pressure  of  the  face  rams 
which  continue  to  support  the  face  until 
the  shield  has  advanced  the  full  length 
of  the  excavation,  when  the  pistons  of 
the  face  rams  are  driven  home  and  the 
face  is  again  supported  by  the  framing 
of  the  shield. 

The  erectors  behind  the  shield  are 
worked  in  the  following  manner.  When 
the  shield  is  pushed  forward  far  enough 
to  allow  of  the  erection  of  a  new  ring  of 
cast-iron  lining,  and  the  five  or  six  seg- 
ments forming  the  invert  of  the  tunnel 
have  been  put  in  place  by  the  simple 
method  of  lowering  them  from  the  level 
of  the  temporary  track,  usually  a  little 
above  the  lowest  table  of  the  shield,  the 
first  segment  which  requires  handling  is 
brought  forward  on  a  trolly  on  the  tem- 
porary track  which  is  usually  carried 
forward  on  to  the  shield,  when  the  erector 
cylinder  J  is  swung  round  so  that  the 
piston  ending  in  the  fork  K  is  directed  at 

the  segment.  The  piston  is  then  extended  by  operating  the  pressure  through  the 
pipe  L,  and  when  sufficiently  extended  the  segment  is  secured  to  the  fork  K 
with  a  loose-fitting  bolt  and  nut.  By  reversing  the  pressure  valves  so  that  the 
water  passes  through  the  pipe  M  instead  of  L,  the  piston  of  the  erector  falls  back, 
lifting  with  it  the  segment  to  be  fixed.  As  soon  as  the  casting  is  clear  of  the  trolley 
one  or  other  of  the  cylinders  R,  R,  are  put  in  operation  and  the  piston  of  the 
main  cylinder  J  pointed  at  the  place  which  the  segment  is  to  occupy.  This  done, 
the  valve  controlling  the  pipe  L  is  opened,  and  the  piston  of  the  cylinder  J  is 
run  out  until  it  presses  the  segment  against  the  inside  of  the  shield  skin,  and 
holds  it  there  until  it  is  bolted  up  to  the  segments  already  erected.  The  bolt  is 
then  taken  from  the  fork  K,  and  the  cylinder  J  swung  round  to  take  the  next 
segment. 

121 


JL. 


ofJPLaJ:forirv. 

FIG.  72.     THE  CENTRAL  LONDON  AND  WATERLOO 

AND  CITY  RAILWAYS. 

Shield  for  Tunnels  of  21  and  23  feet  internal 
diameter. 


TUNNEL    SHIELDS 

The  gang  employed  on  a  shield  of  this  character  is  usually  as  under  :— 

1  ganger, 

8  miners, 

8  miners'  labourers, 

4  general  labourers, 

1  shield  driver, 

1   boy, 

and  the  progress  made  is  usually  from  eighteen  to  twenty-one  rings  per  week,  the 
rings  being  18  inches  wide,  or  say  27  to  31  feet  6  inches  per  week  of  six  days. 

In  London  Clay  these  shields  have  given  very  good  results  :  they  are,  owing 
to  their  comparative  shortness  (in  proportion  to  diameter)  and  number  of  rams 
on  them,  very  easy  to  guide,  and  their  construction  makes  the  work  of  excavation 
fairly  rapid  ;  much  more  so  indeed  if.  measured  by  the  cubic  yards  excavated  than 
with  the  ordinary  Greathead  shield  in  an  11  feet  8  inch  tunnel. 

The  rams  for  driving  the  shields  are  twenty-two  in  number,  and  resemble 
those  used  on  the  smaller  shields,  except  that  the  shoes  or  crossheads  which  bear 
against  the  cast-iron  lining  of  the  tunnel  are  much  smaller,  and  have  the  bearing 
surface  levelled  off,  so  that  they  bear  only  against  the  outside  edge  of  the  castings, 
and  so  minimise  the  risk  of  breaking  the  flanges. 

In  working  a  shield  of  this  size  it  is  usual  to  provide  behind  it  a  timber  staging 
or  gantry  on  wheels  which  runs  on  rails  fixed  on  either  side  of  the  tunnel. 
This  gantry  carries  the  auxiliary  machines  such  as  the  grouting  pans,  pressure 
pumps,  etc.,  and  is  fitted  with  stages  at  different  levels  to  enable  the  grouting 
of  the  tunnel  to  be  easily  carried  out. 

The  temporary  or  working  floor  of  the  tunnel  is  usually  made,  in  the  case  of 
these  large  tunnels,  of  timber.  This  makes  the  invert  below  easy  of  access,  and 
permits  the  caulking  or  pointing  of  the  joints  in  the  lining  to  be  carried  out  rapidly. 

The  Great  Northern  and  City  Railway,  and  the  Kingsway  Subway 
Tunnels  of  the  London  County  Council 

This  railway  connects  the  City  of  London  with  the  Great  Northern  Railway, 
which  it  joins  at  Finsbury  Park  Station,  and  was  opened  in  1904.  The  tunnels 
were  designed  (see  Fig.  34),  to  permit  of  the  passage  through  them  of  the  standard 
gauge  carriages  of  the  Great  Northern  Railway,  and  with  this  object  were  made 
16  feet  in  diameter  instead  of  the  usual  11  feet  6  inches  to  12  feet  of  the  London 
Tube  Railways.  This  increase  in  the  tunnel  diameter  necessitated  an  increase 
in  the  power,  and  additional  stiffness  in  the  framing,  of  the  shield  employed.  The 
former  was  obtained  by  employing  sixteen  rams  instead  of  the  six  or  eight  usually 
put  in  an  11  feet  6  inch  shield,  and  making  these  of  greater  power  ;  the  second 
alteration  consisted  in  the  fitting  of  horizontal  and  vertical  frames  in  the  shield 
similar  to  those  employed  in  the  station  tunnel  shields  just  described,  and 
in  doubling,  for  part  of  its  length,  the  skin,  so  that  the  inner  and  outer  skin,  being 
connected  together  by  plates  and  angle  bracings,  formed  a  very  rigid  casing  to  the 
shield.  As  a  machine  for  working  in  London  Clay,  the  shield  proved  very  satis- 
factory ;  it  was  very  rigid,  and  the  increased  ram  power  enabled  it  to  be  driven 
without  cutting  the  clay  in  front  too  wide  of  the  cutting  edge. 

The  shield  figured  in  Figs.  73,  74,  75  is  not  the  shield  actually  used  in  the 
Great  Northern  and  City  Railway,  but  one,  almost  identical  in  all  details  with  it, 
only  varying  in  diameter,  and  in  which  many  of  the  fittings  of  the  earlier  one, 

122 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

particularly  the  rams  and  hydraulic  erector,  were  used  over  again.  This  shield 
was  used  (1904)  in  driving  two  short  tunnels,  15  feet  in  diameter,  which  are  in- 
tended to  carry  that  portion  of  the  underground  London  County  Council  Tramway 
which  is  being  constructed  along  the  new  Kingsway,  where  it  passes  under  Holborn. 
The  remaining  portion,  or  almost  all  of  it,  of  this  tramway  is  constructed  by  cut  and 
cover  work  ;  but  where  the  tunnels  pass  under  Holborn,  the  shield  method  of 


JfcUf  _£rofib  HLera£Lon.  JCauLf  dross  lectio. 


FIG.  73.     KINGSWAY  SUBWAY,  LONDON. 
Shield  for  Cast  Iron  lined  Tunnel. 

construction  was  adopted,  as  safer  in  view  of  the  heavy  and  valuable  buildings  in 
the  vicinity,  and  of  the  risk  of  damaging  by  settlement  the  sewers  and  numerous 
large  gas  and  water  mains  which  lie  above  the  tunnels. 

The  material  met  with  consisted  of  ordinary  London  Clay,  but  for  some  dis- 

123 


TUNNEL    SHIELDS 

tance  the  top  of  the  tunnel  is  in  ballast  overlying  the  clay,  and  there  it  was  found 
necessary  to  timber  the  face  in  part. 


Kin  O 


ScaLe 

2  3 


JL"fuLcOte 


FIG.  74.     KINGSWAY  SUBWAY,  LONDON. 
Shield  for  Cast  Iron  lined  Tunnel.     Longitudinal  Section. 

The  shield  is  16  feet  inside  diameter  of  skin,  which  consists  of  a  single  thickness 
of  1  inch  plate,  8  feet  3  inches  in  length  and  divided  horizontally  into  six  widths, 

124 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

the  joints,  which  were  covered  with  1  inch  covers,  corresponding  with  the  points 
of  junction  of  the  vertical  and  horizontal  stiffeners.  The  cutting  edge  of  cast 
iron  projects  6  inches  in  front  of  the  skin,  making  the  entire  length  of  the  shield 
8  feet  9  inches,  and  is  divided  into  segments  corresponding  to  the  widths  of  plates 
of  the  skin.  The  outside  of  the  cutting  edge  is  a  little  proud  of  the  outside  of  the 
skin,  but  not  of  the  outside  covers  to  the  joints,  the  front  edges  of  which  are 
bevelled  off  to  avoid  stripping. 

The  cutting  edge,  the  vertical  flange  of  which  is  made  1  foot  5  inches  deep  as 
compared  with  1  foot  1  inch  on  the  shield  from  which  this  one  was  modelled,  is 
bolted  behind  to  the  front  of  a  circular  box  girder  which  performs  the  double  duty 
of  stiffening  the  shield  and  forming  a  series  of  cells  or  compartments  in  which  are 


FIG.  75.     KINGSWAY  SUBWAY,  LONDON. 
Shield  for  Cast  Iron  lined  Tunnel. 


placed  the  shield  rams.  (These  rams  are  not  shown  in  the  general  drawings  of  the 
shield,  but  a  section  of  one  is  given  in  Fig.  76.) 

This  circular  box  is  3  feet  3|  inches  long  inside,  and  1  foot  1  inch  deep,  and  is 
composed  of  \  inch  plates  and  3|  by  3|  by  \  inch  angle  irons. 

The  back  end  plates  of  this  box  are  perforated  by  sixteen  holes,  11  inches 
in  diameter,  through  which  pass  the  pistons  of  the  hydraulic  rams,  and  on  either 
side  of  each  ram  is  a  gusset  plate,  \  inch  thick  with  3|  by  3  J  by  \  inch  angles.  At 
the  junction,  however,  of  the  main  vertical  and  horizontal  frames  of  the  shields, 
the  webs  of  the  frames  themselves,  which  are  continued  through  the  box  girder 
to  the  outside  skin,  form  the  gussets. 

Of  these  frames  two  are  vertical,  and  one  horizontal,  thus  dividing  the  face 

125 


-HP,          ^ 


>> 


PI     • 

£1 

5  Q 


r~     p 

•     M 
C3 


126 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


inch  plates.     The  vertical 


into  six  working  sections.  The  frames  are  made  of 
ones,  which  are  the  same  length 
as  the  circular  box  girder,  are 
stiffened  behind  by  4  by  4  by 
f  inch  angle  irons,  and  in  front 
by  plates  6  inches  wide  and  1  inch 
thick,  having  the  front  edges  be- 
velled off.  They  are  secured  to 
the  skin  of  the  shield  by  6  by  6 
by  f  inch  angles,  to  the  inner 
skin  of  the  box  girder  by  4  by 
4  by  |  inch  angles,  and  to  the 
horizontal  frame,  where  they  in- 
tersect it,  by  four  angles  of  the 
same  dimensions. 

The  horizontal  frame  which 
forms  a  table  or  platform  for 
the  men  working  in  the  upper 
part  of  the  face  consists  of  a 
plate  of  the  same  width  as  the 
vertical  frames,  but  to  it  is  at- 
tached by  bolts,  so  that  it  can 
be  removed,  if  desired,  a  front 
plate  A,  projecting  at  the  centre 
3  inches  beyond  the  cutting  edge 
of  the  shield,  and  supported  by 
brackets  B  fixed  on  the  vertical 
frames.  The  object  of  this  pro- 
jection is  to  afford  support  to 
the  face,  which,  during  the  pro- 
cess of  excavating  a  length, 
can  be  easily  held  up  by  pol- 
ings  supported  by  soldiers  wed- 
ged from  the  front  edge  of  the 
table. 

The  horizontal  frame  is  stif- 
fened at  the  back  by  4  by  4  by 
f  inch  angles,  and  by  vertical 
plates  C,C,  which  also  form  a 
base  of  attachment  for  the  seg- 
ment erector  as  shown  in  Figs. 
73  and  74. 

In  the  vertical  frames,  slots 
D,D,  Fig.  74,  are  cut  to  receive 
bolts  by  which  the  telescopic 

stretchers    E,   E,    can    be    held  ^""XTI"' 

loosely  in  place. 

Inside  the  tail  of  the  shield, 

which  is  built  to  give  1  inch  clearance  round  the  cast-iron  tunnel  lining,  is  rivetted 

127 


TUNNEL    SHIELDS 


a  beading  2£  inches  by  £  inch  to  reduce  the  friction  of  the  shield  when  moving 

forward. 

A  segment  erector  was  employed  with  the  shield  in  the  Great  Northern  and 

City  Railway  work,  but  not  in 
the  Kings  way  tunnels,  and  proved 
a  very  conveniently  arranged 
appliance. 

On  the  plates  C,C,  Figs.  75 
and  79,  are  fixed  two  cylinders 
F,F,  which  have  a  single  double- 
ended  piston  G  carrying  a  rack 
H  (see  Figs.  77  to  81,  on  all  of 
which  the  indicative  lettering  is 
the  same).  This  rack  gears  with 
a  pinion  J,  turning  on  the  cast- 


BB. 

FIG.   78.     KINGSWAY  SUBWAY,  LONDON. 
Erector  fixed  on  back  of  Shield.  Section  online/?,B,  Fig.  77. 


fixed 


on 


the 


ing   K,   which   is 
central  plate  C. 

To  this  pinion  J  is  bolted 
the  casting  L,  also  turning  round 
the  axle  of  K.  This  casting  forms  a  cradle  in  which  rests  the  hinged  casting  M,  in 
which  slides  the  rolled  joist  N,  forming  the  erecting  arm  of  the  machine.  This 
joist  is  9 1  inches  by  5  inches,  the  flanges  being  planed  to  fit  the  casting  M ,  also 
planed.  To  this  casting  is  bolted  the  long  hydraulic  cylinder  P,  to  the  piston  of 
which  the  joist  N,  free  elsewhere,  is  secured  at  R  (Fig.  80). 

One  end  of  this  joist  is  fitted  with  a  hand  S  for   attaching   the  segment  to 
be  lifted.      The  stroke  of  the  piston  in  the  cylinder  P  is  about   4  feet   6  inches, 
and  the  effect  of  working  the  cylinder  is  to  move  the  joist  N  backwards   or  for- 
wards through  the  cast- 
ings M  and  P,  which  form 
a  collar  in  which  it  slides 
freely. 

Rotation  is  imparted 
to  the  arm  by  working 
the  cylinders  F,  F,  caus- 
ing the  rack  H  to  turn 
the  pinion  J,  with  which 
revolves  the  cradle  L,  in 
which  is  fixed  the  collar 
formed  of  M  and  P,  in 
which  the  arm  slides. 

A  useful  arrangement 
is  the  attachment  to  the 
cradle  L  of  the  casting 


T 


Section,  flfl. 


i 


FIG.  79.     KINGSWAY  SUBWAY,  LONDON. 
Erector  on  back  of  Shield.      Section  on  line  A,  A,  Fig.  77. 


M  by  a  pin,  and  an  eye- 
bolt  with  an  adjustable 
screw  shank  (see  Fig.  80), 
so  that  when  the  erector  is  fitted  to  the  shield,  it  is  possible,  by  working  the  eye- 
bolt  screw,  to  adjust  exactly  the  axis  of  the  erecting  arm. 

Hydraulic  power  is  supplied  to  the  two  cylinders  F,  F,  by  independent  pipes 

128 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

on  either  side  of  the  tunnel,  and  to  the  cylinder  P  of  the  revolving  arm  by  flexible 
tubes  connected  to  pressure  pipes  in  the  roof  of  the  tunnel.  This  arrangement  is 
a  better  one,  in  that  in  case  of  failure  in  any  joint,  the  damage  can  be  more  easily 


repaired,  than  the  more  compact  connexions  involving  the  use  of  a  swivel  joint, 
used  for  the  Blackwall  Tunnel  erectors  (see  Fig.  128.) 

The  rate  of  progress  was  very  satisfactory,  three  rings  of  the  tunnel  lining  or 
5  feet    per  day  being   constructed,  and  the  work  was  completed  without   any 

129  K 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 

disturbance  of  the  heavy  buildings  adjoining  the  line  of  the  tunnels.  As  stated 
above,  the  greater  portion  of  the  tunnels  are  entirely  in  the  London  Clay  ;  for 
some  distance,  however,  the  upper  part  of  the  tunnel  cuts  the  gravel  bed  above. 
In  this  gravel  water  is  almost  entirely  absent,  and  even  when  one-half  of  the  shield 
was  gravel,  as  happened  at  one  point,  compressed  air  was  not  required.  The  upper 
portion  of  the  face  was,  however,  timbered,  Fig.  82  showing  the  method  adopted 
when  the  top  of  the  clay  was  just  above  the  axis  of  the  shield.  The  "  guns  "  E,  E, 
shown  in  Fig.  74,  as  hung  in  the  slots  D,  D,  cut  in  the  vertical  diaphragms  of  the 
shield,  were  not  used,  but  instead  the  face  was  secured  by  three  horizontal  stretchers 

a,  a,  a,  Fig.  82,  which  bore  on  byatts  fixed  across  the  tunnel  behind  the  shield, 
somewhat  as  in  the  Baker  Street  and  Waterloo  shield  (see  Fig.  180).     The  crown  of 
the  face  for  a  width  of  about  12  feet  was  close-poled  with  7  inch  by  1|  inch  boards 

b,  b,  b,  which  rested  on  the  cutting  edge  of  the  shield,  their  front  ends  being  sup- 
ported on  the  vertical  polings  c,  c,  c.  Those  vertical  polings  were  set  about  22  inches 


FIG.  82.     KINGSWAY  SUBWAY,  LONDON. 
Shield  for  Cast  Iron  lined  Tunnel.     Timbering  for  Ballast  face. 

in  front  of  the  shield  (the  width  of  a  tunnel  ring  being  20  inches),  and  covered  the 
face  for  a  depth  of  3  feet  6  inches  from  the  crown  of  the  shield.  Below  this  level 
the  face  was  stepped  back  9  inches  and  the  remainder  of  the  gravel  face  was  covered 
by  horizontal  polings  d,  d,  d,  of  the  same  scantling  as  the  upper  ones,  namely  7  inches 
by  1J  inches.  These  polings  were  held  up  by  soldiers  e,  e,  e,  against  which  the 
stretchers  a,  a,  a,  bore,  these  soldiers  also  supporting  the  vertical  polings  by  means 
of  walings  /,  /,  /,  and  chogs  g,  g,  g. 

With  the  face  protected  in  this  manner  the  shield  passed  beneath,  and  within 
a  few  feet  of,  a  newly  constructed  brick  subway,  without  damaging  it. 


A  method  of  supporting  a  face  of  clay  or  other  solid  material  in  a  tunnel 
of  large  diameter  by  means  of  adjustable  iron  stretchers,  no  shield  being 

employed 

In  all  tunnel  work  under  large  buildings,  or  wherever  damage  may  be  caused 
by  settlement,  the  use  of  a  shield  combined  with  an  iron  lining  minimises  the  risk  of 
settlement  and  the  cases  in  which  the  former  may  be  dispensed  with  safely  are  few. 


TUNNEL    SHIELDS 

Cases  do  arise,  however,  where,  owing  to  the  length  of  tunnel  required  being 
very  short,  the  employment  of  a  shield  would,  not  only  on  the  ground  of  expense,  but 
also  on  account  of  the  time  necessary  to  make  a  break-up  for  the  shield,  and, 
afterwards,  to  erect  it,  be  a  doubtful  advantage. 

It  was  for  a  short  length  of  tunnel  of  25  feet  diameter  that  the  arrangement  of 


BcucK  _E 'Legation,. 


FIG.  83.     CENTRAL  LONDON  RAILWAY. 
"  Shield  "  used  in  25  foot  Tunnel  at  Marble  Arch  Station. 

moveable  stretchers  and  cills  shown  in  Figs.  83  and  83A  was  devised  by  Sir  B.  Baker, 
and  gave  very  satisfactory  results. 

The  lower  part  of  the  face  is  held  up  in  the  ordinary  manner  by  a  10  inch  by 
10  inch  cill,  supported  by  two  rakers,  and  by  chogs  at  the  ends  against  the  last 
ring  of  iron  already  erected. 

The  upper  half  is  supported  by  timbers  and  stretchers  so  arranged  that,  as  the 
excavation  proceeds,  the  supports  to  the  face  can  be  advanced,  and  the  new  face 

132 


THE    GREATHEAD    SHIELD    IN    LONDON    CLAY 


held  up  in  such  a  way  that  practically  there  is  no  interval  (as  is  the  case  when 
a  cill  is  moved  forwards),  when  the  clay  is  not  being  sustained  by  the 
frames. 

The  face  is  held  by  six  timbers,  12  inches  by  12  inches  square,  and  from  6  feet 
6  inches  to  7  feet  6  inches  in  length.  These  timbers  are  in  two  rows,  the  one  on 
the  horizontal  diameter  of  the  face,  the  other  halfway  between  the  centre  and  the 
soffit  of  the  excavation.  Each  timber  is  stiffened  behind  by  a  channel  bar  6  inches 
by  3  inches.  To  this  channel  bar  are  ri vetted  two  pairs  of  channel  bars,  also  6 
inches  by  3  inches,  forming  stretchers, 
which  can  be  secured  against  byatts 
or  by  square  bolts  fitting  into  the 
numerous  holes  provided  for  the  pur- 
pose in  the  stretchers  and  driving 
wedges  between  them  and  the 
byatts. 

These  byatts  are  suspended,  for 
convenience  of  handling,  from  the 
roof  of  the  tunnel  by  chains  with 
union  screws.  The  upper  ones,  com- 
posed of  rolled  steel  joists  12  inches 
by  6  inches,  are  made  so  long  that 
when  in  position  their  ends  bear 
against  the  flanges  of  the  tunnel 
lining  ;  the  lower  ones,  composed  of 
rolled  joists  16  inches  by  7  inches,  are 
made  a  little  shorter  than  the  inside 
diameter  of  the  tunnel,  and  take  their 
bearings  against  the  tunnel  lining  by 
catches  or  sliding  bolts. 

These  byatts  can  be  moved  for- 
ward by,  in  the  case  of  the  upper 
ones,  slackening  the  chains  support- 
ing them  until  they  clear  the  iron 
lining  of  the  tunnel,  and,  in  the  case 
of  the  lower  ones,  withdrawing  the 
catches,  when  they  can  be  pushed 
forward,  one  at  a  time,  between  the 
channel  bars  of  the  stretchers. 

The  method  of  working  is  to 
keep  the  upper  part  of  the  face  a 
little  in  advance  of  the  bottom  (see 
Fig.  8 3 A)  and  to  commence  the  erec- 
tion of  each  ring  at  the  top  instead 
of  the  bottom.  As  the  face  is  worked  away,  the  timbers  are  kept  to  it  by 
advancing  the  stretchers  by  means  of  the  bolts  and  wedges  one  hole  at  a  time. 
The  fixing  of  the  cast-iron  segments  follows  close  on  the  excavation,  so  that  there 
are  generally  three,  and  never  less  than  two,  rings  in  course  of  construction  at  once. 
When,  by  the  advance  of  the  work,  the  byatts  require  moving  forward,  the  face  is 
first  made  secure  by  wedging  the  stretchers  hard  against  the  front  byatts,  and 

133 


LongitudinaL  Section 

FIG.  83A.     CENTRAL  LONDON  RAILWAY. 
Shield  "  used  in  25  foot  Tunnel  at  Marble  Arch 

Station. 


TUNNEL    SHIELDS 

then  the  rear  ones,  being  cleared  from  the  segments,  are  slid  forward  between  the 
stretcher  bars,  and  secured  in  their  new  position. 

The  stretchers  are  then  wedged  against  them,  and  the  two  front  byatts  F 
and  G  in  turn  released,  and  moved  forward. 

It  will  be  seen  that,  by  dividing  up  the  support  of  the  face  among  six  inde- 
pendent sets  of  stretchers,  it  was  possible,  after  working  do\vn  the  face  over  a  com- 
paratively small  area,  to  make  it  safe  at  once  instead  of  having  to  finish  the  full 
width  of  the  tunnel  as  if,  say,  the  lower  row  of  timbers  had  been  in  one  length  in- 
stead of  in  three. 

Some  care  was  required  to  keep  the  tunnel  lining  correct  ;  the  tendency  being 
from  the  mode  of  construction  for  the  top  to  come  down,  even  more  than  is  usually 
the  case  in  iron  tunnels,  and  it  was  necessary  always  to  keep  the  horizontal  diameter 
correct  to  dimension  by  means  of  chains  and  union  screws. 


134 


Chapter  V 


THE  SHIELD  IN  WATER-BEARING  STRATA 
THE  ASSISTED  SHIELD 

THE  GREATHEAD  SHIELD  REQUIRES  ADDITIONAL  APPLIANCES  IN  WATER-BEARING  OR  LOOSE 
MATERIAL — THE  CITY  AND  SOUTH  LONDON  RAILWAY  SHIELD  IN  WATER-BEARING  GRAVEL 
—THE  GLASGOW  DISTRICT  SUBWAY  SHIELD — THE  WATERLOO  AND  CITY  RAILWAY  SHIELD 
— DALRYMPLE  HAY'S  HOODED  SHIELD — USE  OF  CLAY  POCKETS  ON  THE  FACE  IN  GRAVEL — 
THE  GLASGOW  HARBOUR  TUNNELS— THE  MOUND  TUNNELS,  EDINBORO' — THE  SIPHONS 
DE  CLICHY  AND  DE  LA  CONCORDE 

IN  the  previous  chapter  it  was  pointed  out  that  the  original  Greathead  shield, 
admirably  adapted  as  it  is  for  work  in  a  material  of  the  homogeneity  and  con- 
sistency of  London  Clay,  is  not  altogether  suitable  for  tunnelling  in  loose,  and  more 
especially  in  loose  water-bearing  material. 

That  its  inventor  was  well  aware  of  this  is  indicated  in  his  own  description  of 
the  works  of  the  City  and  South  London  Raihvay,1  as  he  states  therein  that  he  had 
prepared  a  special  mechanism  for  dealing  with  the  water-bearing  beds  of  gravel 
which  he  anticipated  might  be  met  with,  as  in  fact  they  were,  near  Stockwell  station 
on  that  line,  and  only  abandoned  the  idea  of  employing  a  new  type  of  shield  because 
the  method  of  timbering  in  front  of  the  shield,  tried  as  a  temporary  expedient, 
proved  satisfactory. 

Unfortunately,  an  arrangement  which  was  adopted  on  the  City  and  South 
London  Railway  only  as  a  makeshift  was  copied  in  other  places,  and  proved, 
sometimes  the  reverse  of  satisfactory. 

The  very  excellence  of  the  Greathead  shield  in  the  form  originally  given  it  by 
its  inventor,  and  its  complete  adjustment  to  the  requirements  of  tunnel  work  in 
London  Clay,  make  it  an  imperfect  tool  for  work  in  other  and  looser  material. 

The  nearness  of  the  diaphragm  or  bulkhead  to  the  cutting  edge,  and  the  entire 
absence  of  any  support  to  the  face  of  the  excavation  are  both  serious  defects  in  a 
machine  for  tunnelling  through  loose  material  :  while  the  height  of  the  door  in  the 
diaphragm,  leaving,  as  it  does,  very  little  solid  screen  above  it  which  might,  in  the 
case  of  a  blow  retain  an  air  space  in  which  the  miners  could  shelter,  makes  the  dia- 
phragm entirely  useless,  if  water  comes  in  in  any  quantity  at  the  face. 

To  say  this  is  not  to  depreciate  the  shield,  which  like  most  other  machinery 
has  the  defects  of  its  qualities,  still  less  is  it  to  say  that  in  water-bearing  material 
the  Greathead  shield  is  useless.  It  is,  on  the  contrary,  the  peculiar  merit  of  the 
shield  as  known  since  Greathead  built  the  Tower  Subway  in  1870,  that  it  can  be 
adapted  in  any  circumstances,  and  give,  under  conditions  very  remote  from  those 

1  Proc.  Inst.  C.E.,  vol.  cxxiii.  p.  66.      "  Greathead  on  the  City  and  South  London  Railway." 

135 


TUNNEL    SHIELDS 

for  working  in  which  it  was  especially  designed,  a  cheaper  and  more  secure  and 
incomparably  more  rapid  method  of  tunnelling  than  any  previous  system,  even 
with  the  drawbacks  mentioned  above. 

When,  in  carrying  out  tunnels  in  the  London  Clay,  water-bearing  gravel  is  met 
with,  it  is  the  practice,  if  the  distance  to  be  traversed  in  bad  material  is  not  too  great, 
to  combine  the  use  of  a  Greathead  shield  with  timberwork,  the  whole  operation  of 
working  the  face  and  erecting  the  tunnel  being  carried  out  in  compressed  air. 

If  the  distance  to  be  driven  in  water  justifies  it,  a  special  shield  is  constructed 
for  the  particular  length  of  tunnel  which  requires  special  treatment. 

The  first  method  was  adopted  on  the  City  and  South  London  Railway,  and  in 
the  Glasgow  Circular  Railway  :  the  second  on  the  Baker  Street  and  Waterloo 
Railway,  on  which,  in  the  section  under  the  River  Thames,  a  special  shield  was 


FIG.  84.     CITY  AND  SOUTH  LONDON  RAILWAY. 
Greathead  Shield  with  Timbered  face. 


used  ;  and  both  on  the  Waterloo  and  City  Railway  constructed  at  a  date  inter- 
mediate to  those  two  lines. 

The  method  of  construction  by  means  of  a  Greathead  shield  with  an  advance 
protection  of  timberwork  is  known  as  the  "  assisted  shield  "  method  :  the  shield, 
in  fact,  in  the  earlier  works  carried  out  in  water-bearing  strata,  became  little  more 
than  a  portable  frame  in  which  the  tunnel  was  erected,  advancing  in  a  timbered 
length  of  excavation,  differing  but  little  from  similar  work  in  tunnels  made  with- 
out shields. 

In  tunnels  constructed  recently  in  water-bearing  strata,  the  shields  employed 
have  been  designed  especially  for  the  work  each  one  had  to  do,  and  in  consequence 
each  one  presents  special  features  planned  to  cope  with  the  particular  problems  of 
the  tunnel.  In  most  of  these  shields  the  use  of  timberwork  in  the  face,  though 
rarely  entirely  discarded,  and  hardly  ever  entirely  abandoned,  except  in  special 

136 


THE    SHIELD    IN    WATER-BEARING    STRATA 

cases  like  the  Hudson  Tunnel  in  New  York,  driven  through  liquid  mud,  is  a  subor- 
dinate part  of  the  scheme. 

The  term  "  assisted  shield  "  applies  only  to  the  Greathead  shield,  designed  for 
work  in  clay,  used  in  conjunction  with  a  timbered  face,  or  an  advance  length  of 
timbering.  Of  course,  such  shields  as  those  used  in  the  Greenwich  Tunnel,  and  in 
the  river  portion  of  the  Baker  Street  and  Waterloo  Railway  described  in  Chapter 
VII.  also  require  in  certain  conditions  almost  as  much  timbering  as  an  ordinary 
Greathead  shield.  They  have,  however,  in  themselves  certain  protective  appliances, 
and  so  are  distinct  from  the  simple  Greathead  shield. 

City  and  South  London  Railway 

On  the  City  and  South  London  Railway,  for  the  first  time  in  any  tunnel, 
compressed  air  was  employed  in  conjunction  with  a  shield  at  various  points,  the 
greatest  length  of  tunnel  so  constructed  being  at  Stockwell,  where  the  two  tunnels 
were  driven  through  open  ballast  under  a  head  of  water  of  35  feet ;  the  "  assisted 
shield  "  method  of  tunnelling  was  employed. 

The  shields  used  were  of  course  those  employed  in  other  parts  of  the  line  and 
shown  in  detail  in  Figs.  52,  53,  54  and  55,  and  the  manner  of  using  them  in  con- 
junction with  timbering  is  illustrated  in  Fig.  84. 

Under  an  air  pressure  of  not  more  than  15  pounds,  the  average  rate  of  progress 
per  day  of  two  shifts  was  nearly  5  feet,  a  rate  which,  if  slower  than  the  usual  rate 
of  shield  work  in  other  parts  of  the  tunnel,  is  much  in  excess  of  that  attained  by 
any  of  the  older  methods  of  tunnelling. 

The  airlocks  used  were,  with  the  exception  of  the  first  one,  which  consisted 
of  an  iron  cylinder  built  with  a  brick  bulkhead,  made  by  building  into  the  cast-iron 
tunnel,  a  mass  of  brickwork  having  a  rectangular  opening  in  its  centre  about  3  feet 
9  inches  square.  This  square  was  provided  with  doors  having  cast-iron  frames, 
which  were  built  into  the  brickwork,  forming  an  airlock  about  12  feet  long  (see 
Fig.  85). 

Mr.  Greathead  claimed  for  this  arrangement,  as  compared  with  the  iron-lined 
lock,  the  advantage  "  of  mitigating  the  chilling  effect,  due  to  the  reduction  of 
pressure,  upon  the  men,  hot  from  their  exertions  in  the  warm  compressed  air,  in 
their  egress.  The  brickwork,  absorbing  heat  when  the  lock  is  open  to  the  compressed 
air,  and  parting  with  some  of  it  during  the  reduction  of  pressure  when  closed  against 
the  compressed  air,  is  found  to  preserve  a  more  equable  temperature  than  the  thin 
plates  forming  the  walls  of  the  iron  locks." 

The  advantages  claimed  for  this  kind  of  lock  in  the  passage  just  quoted  do 
not  appear  sufficiently  obvious  to  justify  the  increased  amount  of  brickwork 
necessary,  all  of  which  has  of  course  to  be  cut  out  later,  if  the  full  length  of  the 
lock  is  to  be  of  this  material,  as  against  the  slightly  greater  first  cost  of  an  iron 
or  steel  cylindrical  lock  which  can  be  used  again  and  again.1 

The  manner  of  working  the  assisted  shield  was,  on  this  railway,  as  follows  (see 

Fig.  84).     When  the  shield  was  approaching  the  locality  where  the  open  ballast 

was  known  f  o  exist,  the  ground  in  front  of  and  above  the  shield  was  carefully  probed 

before  each  advance  :    when  the  open  material  was  actually  reached,  the  ordinary 

excavation  of  the  face  was  stopped,  and  the  driving  of  a  small  box  heading  A,  at  the 

very  top  of  the  shield,  started  in  advance.     The  roof  of  this  heading  was  supported 

1  A  brick  lock  was  used  in  the  Siphon  de  Clichy  (1892). 

137 


TUNNEL    SHIELDS 


by  polings  B,  B,  the  rear  ends  of  which  rested  on  the  cutting  edge  of  the  shield. 
The  roof  thus  made  secure,  the  heading  was  gradually  widened  out,  and  the 

circumference  and  face  poled  by  the 
boards  C,  C,  C,  D,  D,  as  the  excavation 
was  carried  down. 

The  face  was  supported  by  means  of 
rakers  E  bearing  against  a  byatt  F  fixed 
across  the  tunnel,  and  bearing  on  the 
flanges  of  the  cast-iron  segments.  It  will 
be  seen  that  this  arrangement  practically 
does  away  with  the  shield  as  a  protection 
to  the  tunnel.  In  the  case  of  a  "  blow  " 
in  the  face  or  roof,  the  doorway  in  the 
diaphragm  or  bulkhead  of  the  shield  can- 
not be  closed,  as  the  rakers  are  in  the  way. 
This  is  the  most  serious  objection  to  this 
method  of  working,  that  it  exposes  the 
miners  at  the  face  to  serious  risk  in  case 
of  even  a  comparatively  small  failure  of 
the  face  work. 

The  best  means  of  diminishing  the 
risk  to  life  caused  by  a  "  blow  "  at  the 
face,  is  the  provision  in  the  tunnel  behind 
the  shield  of  a  hanging  screen,  made  per- 
fectly airtight  at  its  connexions  with  the 
tunnel  lining,  and  coming  as  low  down  in 
the  tunnel  as  the  necessity  of  leaving  room 
for  the  passage  below  of  men  and  materials 
will  permit.  This  screen  makes  the  upper 
part  of  the  tunnel  behind  it  a  kind  of 
diving  bell,  into  which  the  men  could 
escape  in  case  of  accident,  and  along 
which,  by  clinging  to  the  tunnel  lining 
they  could  make  their  way  to  an  emer- 
gency airlock  fixed  in  the  bulkhead  of  the 
ordinary  working  lock.1 

Mr.  Greathead  states 2  that  such 
screens  are  not  possible  in  small  tunnels  ; 
the  author  however  had  one  fitted  up  in 
a  tunnel  of  which  he  was  in  charge,  with- 
out interference  with  the  works  (see  Figs. 
1G8  and  170),  the  diameter  of  which  was 
only  a  few  inches  greater  than  that  of  the 
South  London  one. 

1  The  first  suggestion,  so  far  as  the  Author  is  aware,  of  the  possibility  of  protecting  the 
miners  in  a  tunnel  by  means  of  such  screens,  is  to  be  found  in  the  Scientific  American  of  August 
21,  1880,  in  which  a  Mr.  Van  der  Veyde,  writing  immediately  after  the  disaster  in  the  Hudson 
River  Tunnel,  by  which  twenty  men  lost  their  lives,  suggests  in  a  letter,  accompanied  by  a 
sketch  diagram,  the  employment  of  safety  diaphragms  exactly  as  they  have  many  times  been 
employed  since.  2  Proc.  Inst.  C.E.,  vol.  cxxiii.  p.  108. 

138 


Q     5 
fc    c 

•«!     — 


X    .: 

2£ 
0. 


THE    SHIELD    IN    WATER-BEARING    STRATA 

As  the  support  to  the  face  was  entirely  independent  of  the  shield,  it  was  easy, 
immediately  the  timbered  chamber  was  complete,  to  push  forward  the  shield  into 
it,  when  the  opening  out  of  a  new  chamber  was  commenced.  The  only  time  when 
the  shield  supported  the  face  polings  in  any  way  was  when  it  was  necessary  to  move 
forward  the  byatt  F,  when  for  a  short  time  the  front  polings  were  wedged  against 
the  bulkhead. 

Usually  the  poling  of  the  face  was  complete  down  to  the  invert  of  the  shield,  but 
the  circumferential  ones  did  not  extend  always  quite  round  the  entire  circle,  the  lower 
quarter  of  the  excavation  being  left  open.  Holes  were  drilled  in  alternate  polings 
through  which  lime  grout  under  pressure  was  injected,  and  this  had  the  effect  of  very 
materially  reducing  the  escape  of  air  from  the  face  :  in  fact,  without  this  grouting 
the  air  compressors  employed  would  not  have  been  equal  to  the  task  of  keeping  out 
the  water  over  the  large  area  of  poling  required  by  this  method  of  working. 

In  this  way  the  tunnels  were  driven  in  ballast  beneath  sewers  and  large  water 
mains,  also  in  ballast,  without  causing  any  disturbance  to  them  or  to  the  street 
traffic  above  :  the  general  level  of  the  water  in  the  ground  being  about  35  feet 
above  the  invert  of  the  tunnels. 

The  shield,  when  working  with  a  full  face  of  ballast,  and  with  the  same  material 
below  the  invert,  had  always  a  tendency  to  sink.  This  was  checked  by  using  skids 
of  rails,  and  by  manipulating  the  rams.  In  later  shields  an  increased  number  of 
rams,  and  their  concentration  in  the  lower  half  of  the  shield,  has  obviated  much  of 
this  difficulty. 

The  quantity  of  air  to  be  provided  for  work  of  this  class  was,  at  the  time  these 
tunnels  were  made,  a  matter  of  conjecture  only,  the  experience  gained  in  tunnelling 
with  compressed  air  at  Antwerp  and  New  York  not  presenting  any  analogy  to  tun- 
nelling in  open  ballast,  and  it  was  ultimately  settled  that  a  machine  capable  of 
supplying  1,500  cubic  feet  of  free  air  per  minute  would  be  necessary,  one  tunnel 
only  being  driven  at  one  time. 

The  compressor  laid  down  had  cylinders  18  inches  in  diameter,  with  36  inches 
stroke  working  at  90  pounds  ;  the  air  cylinders,  which  were  tandem  coupled,  hav- 
ing a  diameter  of  26  inches.  These,  when  run  at  fifty  revolutions  of  the  fly-wheel 
per  minute,  actually  delivered  1,660  cubic  feet  of  free  air,  an  inlet  pipe  9  inches  in 
diameter  being  used. 

This  quantity  was  required  until  the  effect  of  grouting  behind  the  poling  boards 
was  tested,  with  the  result  that  while  the  rate  of  progress  was  doubled,  the  rate  at 
which  the  compressor  was  run  dropped  to  from  thirty  to  forty  revolutions  per 
minute,  thus  conclusively  proving  the  efficacy  of  the  grouting. 

The  Glasgow  District  Subway1 

The  tunnels  for  this  work  (1892-5)  were  driven  partly  in  brick  clay,  impervious 
to  water,  and  partly  in  water-bearing  material,  mostly  sand  and  gravel,  and  in  two 
places  actually  passing  under  the  River  Clyde. 

The  work  in  clay  was  carried  out  in  the  same  way  as  on  the  City  and  South 
London  Railway,  and,  judging  by  the  results,  it  would  have  been  better,  apparently, 
had  the  "  assisted  shield  "  work  also  followed  the  earlier  model  more  closely. 

The  engineer  of  this  railway  was  not  however  favourably  impressed  with  the 

1  Proceed.  Inst.  of  Engineers  and  Shipbuilders  in  Scotland,  Jan.  28,  1896.  Simpson  "  On 
Tunnelling  in  Soft  Material." 

139 


TUNNEL    SHIELDS 

Greathead  shield  as  a  tunnelling  machine  in  clay,  and  in  water-bearing  strata  he 
had  put  on  record  his  opinion  that  it  was  of  very  little  use. 

While  some  of  his  criticisms  are  entirely  just  and  reasonable,  there  is  ground 
for  thinking  that  the  difficulties  met  with  in  driving  the  tunnels  for  this  railway 
under  the  River  Clyde  were  in  part  owing  to  the  method  of  timbering  adopted  in 
front  of  the  shield. 

In  Figs.  86,  87,  88  the  details  of  the  timbering  are  shown.  An  advance  head- 
ing A,  6  feet  high  by  4  feet  wide,  was  driven  in  advance.  The  top  polings  of  this 
heading  were  just  high  enough  to  take  under  them  the  3  inch  polings  of  the  chamber 
which  was  subsequently  opened  out.  The  head  and  side  trees  of  the  heading, 
cross-sections  of  which  are  shown  at  B  and  C,  Fig.  88,  were  of  7|  inches  by  3  inch 
timbers,  with  1J  inch  poling. 

When  approaching  the  river  this  heading  was  kept  9  feet  in  advance  of  the 
main  excavation,  which  also  at  that  place  was  made  9  feet  long,  or  sufficient  for 
six  rings  of  tunnel  lining. 


Scale 


FIG.  86.     GLASGOW  DISTRICT  RAILWAY. 
Greathead  Shield  and  Timbered  Heading. 

This  length  in  front  of  the  shield  was  close  polled  from  the  top  to  near  the 
bottom,  the  polings  D,  D,  being  3  inches  thick  :  and  the  face,  with  similar  polings, 
E,  E,  was  held  up  by  9  inch  by  3  inch  horizontal  timbers,  with  two  12  inch  by 
6  inch  soldiers,  F,  F,  stretched  back  to  a  byatt  in  the  tunnel  by  two  8  inch  by 
8  inch  rakers,  G,  G. 

All  the  timber  work  was  well  grouted,  and  when  the  length  was  completed  it 
formed  an  almost  air-tight  barrel  12  feet  3  inches  in  diameter,  into  which  the  shield 
could  be  propelled  and  the  rings  behind  erected  one  by  one. 

This  arrangement  is  certainly  open  to  criticism,  if  only  in  regard  to  the  length 
of  excavation  supported  on  timber. 

In  making  a  brick  tunnel  a  9  foot  length  is  reasonable  enough,  as  heavy  crown 
bars  are  employed  and  all  the  other  framing  is  solid  in  proportion ;  but  in  an  iron 
tunnel,  the  permanent  lining  of  which  can  be  built,  as  in  the  case  under  consider- 
ation, in  successive  lengths  of  18  inches,  it  certainly  appears  to  be  taking  an  un- 
necessary risk  to  open  up  the  ground  for  a  six-ring  length,  with  another  equal  length 
of  heading  in  front  of  it. 

140 


THE    SHIELD    IN    WATER-BEARING    STRATA 

Of  course,  the  shorter  the  chamber, the  more  face  timbering  there  is  to  do;  on  the 
other  hand,  it  is  much  easier  and  safer  to  maintain  a  small  area  of  face  water  tight 
than  a  large  one,  and  in  all  subaqueous  tunnel  work,  safety  is  more  important  than 
speed.  When  actually  working  under  the  River  Clyde,  the  length  of  the  timbered 
chamber  was  reduced  to  6  feet,  and  then  to  4  feet  6  inches,  and  the  advance  heading 
done  away  with,  but  even  the  shorter  length  was  found  too  long  for  safety  by  the 
experienced  contractor  who  ultimately  completed  the  work ;  and  the  engineer,  Mr. 
Simpson  himself,  records  the  serious  difficulties  encountered  under  the  river  near 
St.  Enoch's  when  working  in  the  manner  described,  and  the  improvement  which 
followed  a  change  of  system. 

The  first  tunnel  driven  from  St.  Enoch's  under  the  River  Clyde  had  ten  "  blows  " 
at  the  face  during  the  construction  of  80  feet  of  tunnel,  culminating  in  one  which 
blew  the  whole  timbering  in  front  of  the  shield  up  into  the  river,  and  formed  a  hole 
in  the  bed  24  feet  square  and  16  feet  deep. 

The  cover  over  the  tunnel  varied  from  14  to  29  feet  of  open  material,  and  the 


Scaie 


FIG.  87.     GLASGOW  DISTRICT  RAILWAY. 

Timbered  Face.     (The  Section  is  taken  in  front 
of  the  Shield  in  Fig.  86.) 


FIG.  88.     GLASGOW  DISTRICT  RAILWAY. 
Details  of  heading. 


depth  from  high-water  level  to  the  tunnel  invert  was  from  47  to  55  feet,  which  latter 
is  equivalent  to  a  pressure  of  26- 25  pounds  per  square  inch. 

The  actual  pressure  required  was  at  most  23  pounds,  so  that  the  amount  of 
sand,  etc.,  of  cover  made  very  little  difference  in  the  pressure. 

No  clay  cover  or  blanket,  such  as  has  proved  so  useful  in  similar  cases  elsewhere, 
could  be  used,  owing  to  the  impossibility  of  reducing  the  depth  of  the  waterway  : 
and,  in  consequence,  very  great  difficulty  was  experienced  in  maintaining  the  invert 
of  the  tunnel  fairly  dry  (which  necessitated  a  pressure  greater  than  was  required 
to  dry  the  face  at  the  crown)  and  avoiding  at  the  same  time  the  blowing  out  of  the 
roof  of  the  timbered  chamber  in  front  of  the  shield. 

The  adjustment  of  the  pressure  so  that  the  water  may  not  enter  the  face,  nor 
the  face  blow  out,  is  difficult  at  any  time,  but  the  difficulty  becomes  almost  in- 
superable in  tunnels  of  large  diameter  when  working  in  open  ballast  containing  very 
little  sand  and  under  a  varying  pressure  due  to  the  movement  of  the  tides,  if  clay  is 
not  available  as  a  cover  or  rather  as  a  load  for  holding  down  the  loose  ballast. 

After  80  feet  of  the  first  of  the  two  tunnels  had  been  driven  in  the  gravel  under 

141 


TUNNEL    SHIELDS 

the  river,  and  five  months  had  been  spent  in  this  short  length,  the  contractors  en- 
gaged retired  from  the  work,  and  another  firm  undertook  to  complete  the  section. 

A  considerable  change  was  made  in  the  system  of  timbering  :  the  new  contractor 
at]once  reduced  the  length  of  the  timbered  chamber  to  about  20  inches,  or  sufficient 
for  one  ring  only  of  the  tunnel  lining  ;  the  heading  in  advance  was  abolished,  but 
the  reduction  in  the  size  of  the  chamber  was  not  accompanied  by  any  reduction  in 
the  sizes  of  the  timber  used  ;  and  perhaps  as  important  a  change  as  any  other,  work 
was  carried  on  day  and  night,  Sundays  and  weekdays,  thus  giving  less  time  for  any 
weakness  in  the  timbering  to  develop. 

To  keep  the  roof  secure,  one  very  useful  modification  was  introduced.  Besides 
fastening  the  roof  polings  together  with  iron  dogs,  and  of  course  grouting  well 
behind  them,  the  new  Contractor  introduced  a  variation  in  the  manner  of  holding 


FIG.  89.     GLASGOW  DISTRICT  RAILWAY. 
Bulkhead  and  Airlock. 

the  front  ends  of  the  polings.1  These,  instead  of  being  carried  on  the  ends  of 
the  vertical  polings  of  the  face,  as  is  usual,  were  supported,  and  at  the  same  time 
held  down,  by  a  circular  iron  plate,  \  inch  thick,  bent  to  the  radius  of  the  shield.  To 
this  iron  plate  they  were  secured  by  coach  screws,  and  the  plate  in  turn  rested  on  a 
rib  or  centering  which  at  its  springing  was  securely  fastened  to  a  sill  10  inches 
square  fixed  across  the  centre  of  the  face. 

The  same  arrangement,  without  the  iron  plate,  was  used  at  the  Glasgow 
Harbour  Tunnels  about  the  same  time. 

It  proved  very  effective,  and  was  easier  to  set  up  and  take  down  than  the 
ordinary  poled  supports  had  been. 

For  a  time,  when  working  with  little  cover,  one  ring  per  day  was  done,  but  with 

1  Patent  No.   717  of  1893. 
142 


THE    SHIELD    IN    WATER-BEARING    STRATA 

more  cover,  the  timbered  length  was  made  long  enough  to  admit  of  two  rings  being 
erected  at  a  time,  and  these  lengths  were  also  done  in  twenty-four  hours,  three  eight- 
hour  shifts  being  worked. 

The  average  rate  of  progress  with  this  poling  work  in  front  of  the  shield  was 
about  100  feet  per  month  :   and  the  cost  of  the  tunnel  about  £40  per  yard  forward. 

The  Waterloo  and  City  Railway1 

This  deep  level  railway,  commenced  in  1894  and  completed  in  1898,  connects 
Waterloo  Station,  the  London  terminus  of  the  South-Western  Railway,  with  the 


•    .  \  ^JU-l^LJUL&l^,  '  .    i       ; 

»  .1  '3  ^'>I''-J'*'^_jjl^'*-'g'^-*'1*»'^-^-l  *.*  '*  »  i"*i\V  ,\\  ''^.-   I"  *-    "»'«»*»*''^^  »   ;•"* : '•'•"•"  '•  '• 


FIG.   90.     THE  WATERLOO  AND  CITY  RAILWAY,  LONDON. 
The  Greathead  Shield  and  Timbered  Face. 

City  of  London,  its  eastern  stations  being  situated  under  Queen  Victoria  Street, 
in  close  proximity  to  the  Mansion  House.  For  the  greater  part  of  their  length, 
the  tunnels,  which  are  of  the  iron  lined  type,  are  excavated  in  the  London  Clay, 

1  Proc.  Inst.  C.E.,  vol.  cxxxix.      Dalrymple  Hay  on  "  The  Waterloo  and  City  Railway." 

MS 


TUNNEL    SHIELDS 

and  their  construction  was,  for  the  most  part,  carried  on  on  similar  lines  to  the  tunnels 
of  the  City  and  South  London  Railway  in  the  same  material. 

For  a  considerable  length,  however,  compressed  air  was  employed,  the 
engineers  of  the  Metropolitan  District  Railway,  under  which  the  new  tunnels 
passed  near  Blackfriars  Bridge,  requiring  that  precaution  to  be  taken,  although 
the  work  was  at  that  point  entirely  in  the  London  Clay  ;  and  the  gravelly  nature 
of  the  material  passed  through  between  the  Thames  and  the  Waterloo  terminus, 
and  the  head  of  water  met  with  (about  20  feet)  made  it  necessary  to  employ 
compressed  air  for  a  considerable  distance  on  the  south  side  of  the  river  also.  In 
the  first  case  compressed  air  was  employed  solely  as  a  means  of  giving  support  to 
the  superincumbent  material  ;  in  the  latter  it  was  necessary  in  order  to  enable  the 
work  to  be  carried  on  at  all. 

The  tunnels  under  the  River  Thames  were  driven  through  the  London  Clay, 
there  being  always  a  sufficient  cover  of  this  material  above  to  render  the  use  of 
compressed  air  unnecessary. 

It  was  in  driving  the  tunnels  through  the  water-bearing  gravel  near  to  Water- 
loo Station  that  the  engineers  in  charge  of  the  work  introduced  early  in  1896  an 
improvement  in  the  cutting  edge  of  the  shield  and  in  the  method  of  working  the 
face,  which  has  there  and  elsewhere  since  given  very  satisfactory  results. 

On  commencing  work  with  compressed  air  in  water-bearing  material,  the 
system  of  timbering  employed  in  similar  conditions  in  the  City  and  South  London 
Railway  by  Mr.  Greathead,  who  was  also  one  of  the  engineers  of  the  Waterloo  and 
City  Railway,  was  used  with  slight  variations. 

Figs.  91  and  92  show  the  method  of  timbering  the  face.  A  heading  A,  about 
6  feet  high  by  2  feet  wide,  and  built  in  the  ordinary  way  with  head  and  side  trees, 
was  always  driven  about  two  rings  length,  or  3  feet  4  inches  in  advance  of  the 
chamber  into  which  the  shield  was  ready  to  move. 

Each  time  the  shield  had  moved  forward  the  heading  was  opened  out  to  form  the 
roof  of  the  next  length,  3  feet  4  inches  long,  the  polings  B,  B  were  put  in,  being 
supported  temporarily  on  face  props,  and  at  the  rear  on  the  cutting  edge  of  the 
shield,  and  carefully  grouted  up,  holes  being  drilled  in  them  for  that  purpose,  and 
all  joints  and  openings  being  filled  with  pugged  clay. 

When  the  roof  was  poled,  and  the  excavation  of  the  length  brought  down 
far  enough,  the  first  or  upper  waling  C  was  put  in  position,  and  provisionally  strutted 
against  the  diaphragm  of  the  shield.  On  this  waling  was  erected  a  leading  rib  or 
centre  D  to  support  the  roof  polings.  The  opening  out  of  the  length  was  then  con- 
tinued downwards  until  the  second  waling  E  could  be  got  in,  and  also  strutted 
against  the  shield.  The  face  and  sides  of  the  length  below  the  level  of  the  upper 
waling  C  were  then  poled  and  grouted  as  before,  before  going  down  with  the 
excavation  to  the  third  waling  F,  which  when  put  in  served  also  to  support  the 
piles,  1£  inches  thick,  which  completed  the  closing  of  the  face. 

The  pressure  of  the  face,  which  had  so  far  been  taken  by  the  struts  between 
the  walings  C,  E,  F,  and  the  diaphragm  of  the  shield,  was  on  the  completion  of 
the  length  taken  by  two  rakers  and  two  struts  G,  O,  which  bore  on  a  byatt  J 
fixed  across  the  tunnel  behind  the  shield,  when  the  temporary  props  were  re- 
moved. 

The  face  of  the  timbered  length  or  chamber  having  thus  been  secured  inde- 
pendently of  the  shield,  this  latter  was  pushed  forward  for  the  length  of  two  rings, 
and  in  so  advancing  naturally  disturbed  considerably  the  polings  protecting  the 

144 


THE    SHIELD    IN    WATER-BEARING    STRATA 

roof  and  sides  of  the  chamber,  which  rested  on  it,  and  so  caused  a  considerable 
escape  of  air. 

After  some  months'  experience  of  this  method  of  working,  Mr.  Dalrymple  Hay, 
the  resident  engineer  of  the  railway,  hit  on  the  idea  of  excavating  by  means  of 
a  series  of  pockets  made  in  the  ballast,  and  immediately  filled  with  clay,  a  ring  of 
soft,  airtight  material  into  which  the  cutting  edge  of  the  shield  could  be  driven, 
and  by  the  use  of  which  the  extensive  timber  work  in  front  of  the  shield  necessary 
for  working  the  Greathead  shield  in  ballast  could  be  done  away  with.  He  proposed 


B 


•u;3vJ"-,*«v-  ^-.  -*  V^  •"!.-"""' 


FIG.  91.     THE  WATKKLOO  AND  CITY  KAIL  WAY,  LONDON. 
Timbered  Face.     (The  Section  is  taken  in  front  of  the  Shield — see  Fig.   90.) 

to  combine  this  method  with  a  modification  in  the  shape  of  the  cutting  edge  of 
the  shield,  which  he  proposed  to  make  longer  at  the  top  than  at  the  bottom,  or, 
to  use  the  name  which  custom  has  now  given  to  the  pattern,  he  proposed  a  "  hooded" 
shield.  (The  Frendh  word  viziere  or  vizor  is  a  better  one  than  "  hood  "  to  describe 
the  new  feature.) 

This  arrangement,  while  it  can  hardly  claim  to  be  an  absolutely  novel  idea, 
was  undoubtedly  an  entirely  new  departure  in  shield  work  of  the  modern  kind. 

145  L 


TUNNEL    SHIELDS 

The  clay  pockets  were  a  feature  of  Brunei's  shield  work  1  and  only  a  few  months 
previously  in  the  tunnel  under  the  Seine  of  the  Siphon  de  la  Concorde  at  Paris,  a 
Greathead  shield  was  employed  which  the  French  engineer  in  charge  of  the  works 
had  fitted  with  a  sliding  roof  or  "  parapluie,"  which  could  be  advanced  about 
1  foot  6  inches  in  front  of  the  cutting  edge  of  the  shield,2  and  appears  to  have  given 
satisfactory  results.  This  tunnel  also  was  driven  through  sand  and  gravel  for  the 
greater  part  of  its  length,  and  through  a  chalk  bed  broken  up  by  many  fissures. 

The  first  attempt  made  by  Mr.  Hay  to  extend  the  front  of  the  shield  was  on 
similar  lines  and  is  shown  in  Fig.  92.  It  consisted  simply  of  two  plates,  2  feet 
9J  inches  wide  and  \  inch  thick,  curved  to  the  radius  of  the  cutting  edge  of  an 
ordinary  Greathead  shield,  and  extending  round  it  for  a  length  equal  to  about  one- 
third  of  its  circumference.  The  plates  were  secured  in  front  of  the  shield  by  six 
gussets  which  were  rivetted  to  the  plates,  and  bolted  to  the  cast-iron  cutting  edge 
and  the  vertical  diaphragm  of  the  shield  as  shown  in  the  figure.  Grout  holes  were 
provided  in  the  plates  forming  the  hood. 


r 


FIG.  92.     THE  WATERLOO  AND  CITY  RAILWAY,  LONDON. 
Dalrymple-Hay's  Hood  on  a  Greathead  Shield. 


At  first  it  was  proposed  to  loosen  the  ballast  in  front  of  the  cutting  edge  of 
the  hood  by  the  miners  working  it  with  timber  dogs,  but  a  very  short  trial  showed 
that  this  was  impossible,  and  Mr.  Hay  determined  to  try  the  effect  of  removing  an 
annular  space  in  front  of  the  cutting  edge  by  making  a  succession  of  pockets,  each 
pocket  when  made  being  filled  with  well-pugged  clay.  The  ring  so  made  was  got 
out  a  little  larger  than  the  outside  of  the  hood,  so  that  when  the  latter  advanced 
into  it,  a  skin  of  clay  from  1  to  2  inches  thick  remained  outside  the  skin  of  the 
shield,  and  served,  as  the  shield  advanced,  in  some  measure  as  an  airtight  coating 
outside  the  tunnel  lining. 

A  more  detailed  description  of  the  process  is  given  in  dealing  with  the  first 
hooded  shield  employed  later  by  Mr.  Hay  (p.  148). 

This,  in  being  tried,  was  found  quite  feasible,  and  the  amount  of  air  to  be 
pumped  was  considerably  diminished,  while  the  cost  of  the  tunnel  gang  was  reduced 
and  the  rate  of  progress  increased.  But  the  system  had  hardly  got  into  working 
order  when  the  shield  showed  signs  of  failure  due  to  the  extra  pressure  on  the 
extended  front,  and  the  fear  of  the  shield  collapsing  altogether  caused  the  contractors 

1  Weale's  Quarterly  of  Engineering,  1846,  vol.  v. 

2  Legouez,  Emploi  du  Bouchier,  pp.  278,  280. 

146 


^~-J~.-^~-<  \':~'\  J*    -  *.5*V     ',  ^•* 


. - .   -          -~  -•-   .  ••• .. 


''''          '    '  '  "  "' 


FIG.  93.     WATERLOO  AND  CITY  RAILWAY,  LONDON. 
Dalrymple-Hay's  Hooded  Shield.     Longitudinal  Section. 

147 


TUNNEL    SHIELDS 

to  remove  the  hood,  and  revert  to  the  older  system  of  timbering  in  advance.  The 
great  economy  effected,  particularly  in  the  supply  of  air  required,  by  the  use  of 
the  hood  and  the  clay  pockets,  was,  however,  so  obvious,  that  a  little  later  they 
employed,  in  another  part  of  the  railway,  a  specially  made  shield  embodying  Mr. 
Hay's  ideas,  in  place  of  the  provisional  one  previously  used.  This  shield  is  shown 
in  Figs  93,  94,  95.  It  consists  of  a  cylindrical  skin  formed  of  f  inch  plates,  which, 
contrary  to  the  usual  practice  in  shields  of  this  size,  was  not  in  one  piece  from 
front  to  back  of  the  shield,  but  there  was  a  circumferential  butt  joint  at  A,  4  feet 
4  inches  from  the  tail  of  the  shield,  and  5  feet  2  inches  from  the  front  of  the  hood. 
This  skin  extended  to  the  cutting  edge  B,  there  being  no  cast-iron  cutting  edge, 
but  one  composed  of  plates  making  a  total  thickness  of  2J  inches  for  the  greater 
part  of  the  circumference,  and  of  1|  inches  at  the  invert.  This  cutting  edge 
extends  3  feet  9  inches  in  front  of  the  vertical  diaphragm  E  of  the  shield,  except  at 
the  bottom,  where  it  is  cut  away  (at  C)  so  that  it  is  only  1  foot  9  inches  long.  A 
projecting  edge  of  this  length  would  not,  of  course,  be  sufficiently  stiff  in  itself  to 
resist  the  pressure  of  the  ground  above,  but  it  and  the  shield  generally  is  stiffened 
by  a  circular  box  girder  D  immediately  in  front  of  the  diaphragm,  to  which  and 
to  the  cutting  edge  plates  it  is  rivetted. 

The  overhang  of  the  cutting  edge  beyond  the  box  is  further  supported  by 
nine  gussets  F,  F,  formed  of  plates  and  angle  irons. 

The  arrangement  of  the  hydraulic  rams  (not  shown  in  the  figures)  and  of  the 
ram  castings  is  of  the  usual  type.  They  were,  in  fact,  taken  from  the  Greathead 
shield  on  which  the  experimental  hood  was  tried. 

The  shield  measured  over  all  9  feet  6  inches  in  length,  and  13  feet  9  inches 
in  outside  diameter.  It  was  thus  about  2  feet  6  inches  longer  than  the  ordinary 
Greathead  shield,  the  cost  being  about  the  same. 

It  will  be  seen  that  the  overhanging  cutting  edge  formed  a  comparatively 
roomy  chamber  in  which  the  miners  could  work  in  safety,  and  it  is  further  claimed 
for  the  hood  that  it  admits  of  the  ballast  in  the  face  being  entirely  removed  in 
front  of  the  cutting  edge  at  the  invert,  and  so  facilitating  the  advance,  and 
removing  for  that  part  of  the  face  the  risk  of  the  cutting  edge  being  deformed 
by  meeting  boulders  or  large  stones. 

The  other  innovation  made  in  combination  with  the  hooded  shield  was  the 
employment  of  tempered  clay  in  front  of  and  around  the  cutting  edge  for  the  double 
purpose  of  limiting  the  escape  of  air  at  the  same  time  that  the  amount  of  timber- 
ing in  front  was  reduced,  and  of  preventing  in  some  measure  the  settlement  of 
the  ground  above.  In  both  directions  the  arrangement  was  satisfactory  ;  the  amount 
of  air  required  to  dry  the  face  and  the  cost  of  labour  were  reduced  ;  there  was  very 
little  settlement  in  the  buildings  above,  and  further,  the  rate  of  progress  was  about 
24  per  cent,  quicker  than  with  the  timbered  heading  method  of  work. 

The  method  of  employing  the  clay  was  as  follows  : — Commencing  at  the  crown 
of  the  shield,  a  hole  about  15  inches  wide  and  22  inches  long  was  formed  in  the 
ballast  in  front  of  the  cutting  edge,  by  raking  it  out  with  a  timber  dog,  or  a  short 
bar,  and  at  once  filled  up  with  tempered  clay,  the  outside  of  the  hole  being  about 
2  inches  above  the  cutting  edge  (see  G,  Fig.  93).  Another  hole  of  like  dimensions 
was  then  commenced  at  the  side  of  the  first  and  clayed  up,  and  the  process 
repeated  until  finally  a  ring  of  tempered  clay  was  formed  in  front  of  and  extending 
some  2  inches  outside  of  the  cutting  edge  of  the  hood.  The  clay  ring  was  not  carried 
below  the  hood. 

148 


THE    SHIELD    IN    WATER-BEARING    STRATA 

Care  was  required  in  making  those  pot  holes,  and  it  was  necessary  to  clay 
them  up  immediately  they  were  made,  but  the  operations  were  simple  enough  in 
themselves. 


LTI  cU:  cij-ttirifi  eetge 
<,ST>y  forrnisig  fiat  fioiea  in, 


FIG.  94.     WATERLOO  AND  CITY  RAILWAY,  LONDON. 
Dalrymple-Hay's  Hooded  Shield.     Half  Cross  Section. 

In  Fig.  93,  the  clay  ring  G  is  shown  as  complete,  and  the  shield  ready  to  be 
pushed  forward.     As  the  shield  advanced  to  the  position  shown  by  the  dotted  line 

149 


\     %t? 
\      3& 


FIG.  95.     WATERLOO  AND  CITY  RAILWAY,  LONDON. 
Dalrymple-Hay's  Hooded  Shield.     Timbered  Face. 


150 


THE    SHIELD    IN    WATER-BEARING    STRATA 

H,  H,  the  cutting  edge  of  the  hood  buried  itself  in  the  tempered  clay.  The  clay 
ring  being  got  out  some  inches  outside  the  cutting  edge,  a  layer  of  clay  was  left 
behind  as  the  shield  advanced,  and  is  said  to  have  remained  in  a  comparatively 
continuous  layer,  not  only  on  the  shield  but  outside  of  the  tunnel.  This  of 
course  had  the  effect  of  greatly  reducing  the  usual  loss  of  air  between  the  tail  of 
the  shield  and  the  last  tunnel  ring,  and  also  at  the  joints  and  grout  holes  of  the 
tunnel. 

This,  and  the  fact  that  the  method  of  working  did  away  entirely  with  the 
advance  top  heading,  doubtless  reduced  the  escape  of  air  by  50  per  cent,  as 
compared  with  the  earlier  system. 

The  timber  work  of  the  face  is  all  done  under  the  shelter  of  the  hood, 
and,  there  being  no  heading,  the  amount  of  work  to  be  done  is  comparatively 
small. 

Each  time  the  shield  was  pumped  forward  and  a  new  ring  of  the  tunnel 
lining  erected,  the  setting  forward  of  the  timbered  face  was  proceeded  with 
before  the  making  of  the  clay  ring  already  described  was  put  in  hand.  The  poling 
boards  at  the  upper  part  of  the  face  were  advanced  to  the  line  of  the  cutting  edge 
and  the  walings  J1  J2  J3  J3  successively  put  in  as  the  face  was  worked  down  in 
the  same  manner  as  in  the  timbered  lengths  already  described.  The  walings 
were  stretched  temporarily  to  the  diaphragm  of  the  shield  by  the  struts  K ,  K 
indicated  in  dotted  lines  in  Figs.  93  and  95.  When  the  face  was  secured  in  this 
manner,  and  well  grouted  up,  the  clay  ring  was  formed  as  already  described,  and 
the  length  bottomed  up. 

On  the  completion  of  the  timbering  and  excavating  of  the  face,  the  walings, 
which  until  then  were  stretched  back  to  the  shield,  were  supported  by  soldiers  L, 
and  struts  and  rakers  M ,  M  bearing  on  a  byatt  fixed  across  the  tunnel,  thus 
leaving  the  shield  free  to  move  forward  again,  without  disturbing  the  timbered 
face. 

It  will  be  noted  that  this  system,  although  a  great  improvement  on  that 
previously  employed,  was  still  open  to  the  objection  that  by  the  use  of  struts  and 
rakers  passing  through  the  door  of  the  shield,  when  it  was  advancing — that  is,  when 
collapse  of  the  face  was  most  likely  to  occur — no  possibility  existed  of  closing  up 
the  front  of  the  shield  in  case  of  accident. 

Owing,  too,  to  the  "  overhang  "  of  the  hood  of  the  shield,  the  tendency  which 
every  shield  has  when  going  through  water-bearing  ballast,  to  sink  as  it  advances, 
was  perhaps  increased,  and  no  doubt  Mr.  Hay's  use  of  a  clay  lining  outside  of  the 
shield,  by  its  tendency  to  yield,  assisted  in  the  movement.  This  was  counteracted 
by  the  use  of  timber  skids  and  iron  plates  in  the  bottom  of  the  invert. 

Another  advantage  which  the  use  of  the  clay  ring  possesses  when  it  is  made, 
as  was  the  case  in  the  Waterloo  and  City  Railway  work,  sufficiently  large  to  leave 
a  clay  ring  some  2  inches  thick  round  the  shield,  is  to  increase  the  facility  of  handling 
the  shield  on  a  curve.  This  in  Mr.  Hay's  shield  was  further  made  easier  by  fixing 
on  the  outside  of  the  skin,  and  on  either  side  of  the  shield,  a  f-inch  plate  N  (Fig.  94) 
extending  backwards  from  the  cutting  edge  a  distance  of  3  feet  9  inches,  and 
reaching  above  and  below  the  horizontal  diameter  of  the  shield  about  5  feet 
9  inches.  The  effect  of  the  extra  thickness  thus  provided  in  the  front  part  of  the 
skin  of  the  shield  was  to  make  a  wider  cut  as  the  shield  advanced  in  which  the  tail 
could  turn  ;  and  as  a  matter  of  fact  the  shield  was  so  driven  round  a  curve  of  320 
feet  radius  without  difficulty. 

151 


TUNNEL    SHIELDS 


The  Glasgow  Harbour  Tunnels  (1890-93) 

The  Glasgow  Harbour  Tunnels  1  were  constructed  under  the  River  Clyde,  and 
are  peculiar  in  that  they  are  all  connected  to  single  shafts  on  either  side  of  the 
river  and  are  so  close  together  that  there  is  in  places  barely  2  feet  of  intervening 
material  between  them.  The  three  tunnels  were  provided  to  afford  the  required 
traffic  facilities  and  at  the  same  time  keep  the  diameter  of  the  tunnels  as  small 
as  possible. 

The  vehicular  traffic  going  south  has  a  separate  subterranean  passage,  and 
there  is  also  one  for  the  traffic  going  north,  while  the  central  tunnel  is  for  passengers 
only  (see  Fig.  96). 


iSoti  ff/Jief 


FIG.  96.     GLASGOW  HAKBOUK  TUNNELS. 
General  Plan  and  Section  of  Works. 

The"  diameter  of  the  part  of  the  tunnel  under  the  river,  built  in  cast-iron 
segments,  is  16  feet  ;  that  under  the  quays,  where  there  is  boulder  clay,  is  built  of 
brick  arching,  and  is  18  feet  in  diameter.  At  their  highest  points  the  tunnels  are 
15  feet  below  the  bed  of  the  river,  thus  leaving  ample  room  for  future  dredging 
operations,  and  35  feet  and  46  feet  respectively  below  low  and  high  water  levels. 
The  shaft  on  the  north  side  is  about  400  feet  west  of  Finnieston  Street  and  170 
feet  from  the  quay  wall,  while  the  shaft  on  the  south  side  adjoins  the  Govan  Road, 


1  Engineering,  May  10  and  31,  and  June  14  and  28,  1895,  from  which,  by  the  courtesy  of 
the  Editor,  the  illustrations  are  reproduced. 

152 


THE    SHIELD    IN    WATER-BEARING    STRATA 

and  is  120  feet  from  the  quay  wall.  As  the  river  is  415  feet  wide,  the  length  of 
tunnel  from  shaft  to  shaft  is  just  over  700  feet.  Both  shafts  are  round,  and  76  feet 
in  diameter.  The  shaft  on  the  north  side  of  the  river  is  72  feet  6  inches  deep,  and 
that  on  the  south  side  75  feet  6  inches  deep.  In  each  shaft  there  are  six  elevators, 
three  for  lifting  and  three  for  lowering,  but  any  and  all  can  be  used  either  for  lifting 
or  lowering  when  required.  They  are  for  vehicles.  The  passenger  tunnel  pierces 
the  shaft  34  feet  from  quay  level,  with  flights  of  stairs.  From  the  shaft  it  is  on  a 
decline  of  1  in  3. 

The  work  of  constructing  the  shafts  was  started  on  February  3,  1890,  and  the 
south  shaft  was  first  taken  in  hand.  The  walls  of  the  shaft  consist  for  the  greater 
part  of  their  depth  of  an  inner  and  an  outer  lining  of  J-inch  cast-iron  segments  braced 
together  with  wrought  iron  T  bars  3  inches  deep  by  \  inch  thick,  the  intervening 
space  being  filled  with  concrete  consisting  of  five  parts  of  sand  and  broken  stones 
to  two  of  cement.  The  total  thickness  is  4  feet.  The  upper  and  lower  parts  of  the 
completed  shaft  are  entirely  of  brickwork.  The  upper  soil  of  sand  was  removed 
to  a  depth  of  14  feet,  when  water  was  reached.  A  double  ring  of  segments,  each 
of  2  feet  depth,  was  then  built  on  a  cutting  edge.  Other  rings  were  built  and  filled 
with  concrete  as  the  excavation  proceeded,  pumping  arrangements  being  mean- 
while introduced  to  deal  with  the  water.  With  about  thirty  miners  employed  the 
shaft  was  carried  down  at  about  the  rate  of  8  feet  per  month  so  long  as  the  material 
met  with  was  sand  only.  At  a  depth  of  48  feet,  however,  boulder  clay,  the 
surface  of  which  was  much  higher  on  one  side  of  the  shaft  than  on  the  other,  was 
found. 

Considerable  trouble  was  experienced  in  consequence  in  keeping  the  shaft 
level  and  true,  but  by  employing  pig  iron  as  kentledge  in  the  side  where  the  clay 
was  highest,  and  by  lubricating  the  outside  at  the  same  place  with  water  by 
means  of  a  trench  dug  round  the  caisson,  it  was  sunk  into  the  clay  without 
deformation. 

When  the  shaft  was  well  into  the  clay,  all  round  the  cutting  edge  and  the 
remaining  21  feet  of  depth  was  built  by  underpinning  in  brickwork  4  feet  thick. 
This  was  done  without  difficulty  except  on  the  south  side,  where  sand  was  found. 
This  was  dealt  with  by  driving  piles  inside  the  caisson  to  a  depth  of  in  seme  cases 
40  feet,  and  when  it  was  dry,  excavating  the  sand  behind  them  and  underpinning  in 
short  lengths. 

The  bottom  being  mainly  clay,  a  concrete  invert  2  feet  in  thickness  was 
considered  sufficient. 

The  north  shaft  was  sunk  under  more  difficult  conditions,  the  material  being 
fine  sand,  which  at  a  very  small  depth  beneath  the  surface  was  waterlogged.  The 
rate  of  progress  was  about  6  feet  per  month,  and  the  sinking  of  the  caisson  was  easy, 
but  of  course  expensive  pumping  plant  was  required,  the  amount  raised  reaching 
1,500  cubic  feet  per  minute.  When  sinking  in  this  loose  material,  however,  the 
cast-iron  segments  of  the  lining  showed  indications  of  parting,  and  ultimately  a 
second  or  inner  lining  also  of  cast  iron  and  secured  to  the  original  caisson,  the  spaces 
between  them  being  filled  with  concrete,  was  constructed. 

The  use  of  cast  iron  in  a  double-skinned  caisson  does  not  appear  to  have  been 
satisfactory.  In  part  this  may  have  been  due  to  imperfect  bracing,  but  the  main 
objection  to  the  use  of  a  caisson  with  two  skins  formed  of  cast-iron  segments  is  that 
in  the  event  of  irregular  sinking  in  bad  material  the  stresses  set  up  in  the  caisson 
may  be  tensional  and  not  merely  compressional,  as  they  are  in  shafts  sunk  in 

153 


TUNNEL    SHIELDS 

good  material.  In  such  conditions  a  frame  of  cast-iron  segments  bolted  each 
to  its  neighbours  is  the  worst  kind  of  lining  to  a  caisson. 

The  invert  of  this  shaft  was  covered  with  concrete  10  feet  thick,  the  cast-iron 
lining  being  carried  down  to  the  bottom  and  no  brickwork  used. 

Of  the  three  tunnels  which  connect  the  tAvo  shafts,  the  two  outer  ones  were 
made  level  from  shaft  to  shaft,  being  reached  by  lifts  which  are  large  enough  to 
take  vehicles  of  any  size,  these  two  tunnels  being  reserved  for  wheeled  traffic. 

The  central  tunnel  leaves  each  shaft  about  35  feet  from  the  top,  and  descends 
to  below  the  river  bed  on  gradients  of  1  in  3,  stairways  being  provided  for  foot 
passengers  both  in  the  shafts  and  in  the  inclines  of  the  tunnels. 

Owing  to  the  fact  that  the  boulder  clay  met  with  in  the  south  shaft  extended 
for  a  considerable  distance  under  the  river,  it  was  possible  to  construct  the  first 
portion  of  the  tunnels,  starting  from  that  shaft,  in  the  ordinary  manner  in  timbered 
lengths,  the  tunnel  being  lined  with  brick,  the  internal  diameter  being  18  feet. 
This  size  enabled  the  shield  for  driving  the  cast-iron  lined  tunnels  1 6  feet  in  diameter 
under  the  river  to  be  taken  through  it  with  ease. 


FIG.  97.     GLASGOW  HARBOUR  TUNNELS. 
Shield. 


Compressed  air  was  employed  in  all  three  tunnels  under  the  river,  and  shields 
also,  these  latter  being  Greathead  shields,  resembling  in  some  respects  the  Glasgow 
Subway  shield,  but  of  greater  strength.  Two  types  of  shield  were  used,  the  second 
of  which  is  shown  in  Fig.  97. 

In  was  17  feet  3  inches  in  diameter  and  8  feet  6  inches  in  length  over  all.  The 
skin  was  composed  of  two  plates  A  inch  in  thickness,  and  there  was  no  cast-iron 
cutting  edge,  as  was  usual  with  all  the  Greathead  shields  of  that  date. 

The  plate  diaphragm  placed  1  foot  1  inch  back  from  the  front  edge  was  of  \  inch 
plates,  and  was  pierced  by  two  doors  6  feet  6  inches  high  and  4  feet  2  inches  wide. 
These  doors  could  be  closed  by  sliding  doors,  like  the  ones  on  the  Glasgow  Subway 
shields  (see  Fig.  60).  These,  however,  were  never  used.  The  special  feature  of 
this  diaphragm,  however,  was  the  manner  in  which  it  was  stiffened  by  horizontal 
and  vertical  girders  behind  it.  These  girders,  formed  of  £-inch  plates,  stiffened  in 
the  case  of  the  vertical  ones  A,  A,  A,  Fig.  97,  by  channels  6  inches  by  3  inches 
by  3  inches  by  \  inch,  and  in  the  horizontal  ones  B,  B,  by  angles  5  inches  by 
5  inches  by  f  inch,  gave  great  rigidity  both  to  the  diaphragm  and  to  the  skin  of 
the  shield. 


154 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  rams  were  thirteen  in  number  and  7  inches  in  diameter,  fitted  in  a  cast-iron 
ring  of  the  same  length  as  the  diaphragm  girders  were  wide.  The  tail  of  the  shield 
was  2  feet  11  inches  long. 

The  greatest  pressure  employed  in  the  tunnel  never  exceeded  18  pounds,  and 
was  usually  much  less.  The  compressed  air  plant  was  therefore  of  comparatively 
limited  capacity. 

The  bulkheads  and  airlocks  employed  were  of  two  types.  In  the  first  the 
bulkheads  were  of  brickwork,  the  total  thickness  being  19  feet,  and  the  lock  being 
simply  an  opening  5  feet  by  3  feet  6  inches  left  in  the  wall,  and  closed  by  cast  metal 
doors  fitted  to  frames  similar  to  those  employed  in  the  Mersey  Tunnel  lock  (see 
Fig.  149). 

In  the  second  the  bulkhead  consisted  solely  of  a  curved  diaphragm  A  fitted 
to  the  cast-iron  lining  of  the  tunnel  (see  Fig.  98),  with  the  convex  side  towards  the 
pressure  chamber,  and  stiffened  by  seven  vertical  gussets  B,  B.  Secured  to  this 
diaphragm  on  the  outside — that  is,  in  the  ordinary  atmosphere — was  a  lock  about 
13  feet  long,  5  feet  7  inches  high  and  4  feet  3  inches  wide,  rectangular  in  shape,  the 
casing  being  made  of  buckled  plates,  with  the  convex  side  inwards. 


FIG.   98.     GLASGOW  HARBOUR  TUNNELS. 
Built  from  Bulkhead  and  Airlock. 


This  arrangement  proved  very  satisfactory,  and  its  removal,  when  done  with, 
was  easy. 

The  work  of  driving  the  tunnels  was  carried  out  successfully,  though,  com- 
pared with  some  later  works,  the  rate  of  progress  appears  somewhat  slow. 

The  first  start  was  made  with  the  down-stream  tunnel,  and  as  this  was  for 
some  distance  entirely  in  clay,  the  tunnel  was  for  some  distance  constructed  with- 
out shield  or  air  pressure,  and  indeed  without  iron  lining,  brick  being  employed. 

This  brick-lined  tunnel  was  made,  as  stated,  about  18  feet  in  diameter,  thus 
enabling  the  shield  for  the  16  feet  tunnel  to  pass  through  it.  A  length  of  60  feet 
was  driven  without  air. 

The  compressed  air  plant  was  started  on  June  1,  1891  ;  but  as  long  as  the  work 
was  through  boulder  clay  little  pressure  was  required.  The  experience  through- 
out the  work  was  that  the  rate  of  progress  was  not  affected  by  the  fact  that  the 
men  had  to  work  under  air  pressure  ;  but  the  pressure  seldom  exceeded  10  pounds. 
When  the  men  were  working  without  air  pressure,  the  progress  was  12  lineal  yards 
for  the  first  month,  May,  and  for  the  June  following,  when  the  air  pressure  was 
first  started,  14,  lineal  yards  ;  in  July  it  was  17  yards.  In  August,  when  operations 

155 


TUNNEL    SHIELDS 

were  half  in  clay  and  half  in  sand,  the  progress  was  19  yards,  and  the  average  in 
the  "sand  under  air  pressure  about  20  yards  a  month.  The  air  pressure  varied 
greatly,  from  18  pounds  when  the  sand  was  first  reached,  to  2J  pounds  when  the 
north  shaft  was  pierced.  The  highest  pressure  was  reached  crossing  the  middle 
line  of  the  river,  when  there  was  a  little  wet  sand.  At  high  tide  there  was  a  head 
of  60  feet,  and  at  low  water  of  50  feet  over  the  tunnels. 

The  east  tunnel  was  started  from  the  south  shaft,  in  boulder  clay,  as  the  west 
tunnel  was  nearing  the  north  shaft  ;  but  when  the  east  tunnel  got  into  the  sand,  it 
was  found  desirable  to  add  to  the  air  compressing  plant,  and  ultimately  it  was 
found  unadvisable  to  continue  working  the  two  tunnels  simultaneously,  even  with 
the  increased  supply  of  air,  and  work  in  the  west  tunnel  was  suspended,  and  the 
construction  of  the  eastern  one  pushed  on. 

In  February  1892,  when  the  airlock  for  the  east  tunnel  had  been  completed 
and  the  air  pressure  was  ready  to  be  turned  on,  a  "  sand  back  "  discovered  itself 
in  the  boulder  clay,  and  the  water  from  the  river  came  flowing  into  the  tunnel. 
It  was  on  a  Saturday  night,  so  no  men  were  in  attendance,  but  the  airlock  door  was 
closed,  so  that  the  tunnel  only  was  flooded.  The  difficulty,  however,  was  over- 
come, for  with  a  15-pound  air  pressure  the  tunnel  was  blown  completely  dry  in 
twenty-four  hours.  That  was  the  only  incident  in  the  boring  of  the  east 
tunnel,  which  was  completed  in  November  1892,  and  the  men  went  into  the  west 
tunnel  and  completed  it,  as  already  described,  in  February  1893. 

The  centre  tunnel  then  only  remained  to  be  driven,  and  it  was  completed 
without  a  hitch  by  November  1893,  the  rate  of  progress  being  greater  than  in  the 
other  two  tunnels.  In  one  month  30  lineal  yards  were  driven  in  the  middle  of  the 
river,  and  in  another  month  25  yards. 

The  cost  of  excavation  under  air  pressure  in  sand  was  about  105.  per  cubic 
yard,  including  every  operation.  The  men  engaged  on  the  work  were  well 
paid,  being  almost  all  skilled  labourers.  The  miners  got  about  85.  a  day. 
The  total  cost  per  yard  of  iron-lined  tunnel,  16  feet  in  internal  diameter,  under 
the  river  was  from  £80  to  £85  per  yard  of  length.  This  price  appears  somewhat 
high,  but  the  comparatively  short  length  of  tunnel,  only  700  yards  in  the  three  tunnels 
altogether,  makes  the  charge  per  yard  for  shields  and  air  plant  very  heavy  in 
proportion  to  the  total  working  charges. 


The  Mound  Tunnels,  Edinburgh 

The  Mound  Tunnels  at  Edinburgh  were  constructed  in  1893-4  by  the  North 
British  Railway  Company  as  part  of  a  scheme  for  improving  the  traffic  facilities 
at  the  Waverley  Station.  Two  tunnels,  each  16  feet  4  inches  in  diameter,  and 
750  feet  long,  were  constructed  in  cast  iron  under  the  "  Mound,"  which  is  an  arti- 
ficial hill  connecting  the  old  and  new  towns,  and  lies  across  the  valley  in  which 
the  railway  runs.  From  the  fact  that  the  hill  consists  of  made  earth  and  that 
it  carries  some  important  public  buildings — an  art  gallery,  etc. — it  was  decided  to 
use  compressed  air  to  hold  up  the  excavation.  Only  a  low  pressure  was  used, 
some  15  pounds  per  square  inch,  but  the  work,  which  was  carried  out  with 
complete  success,  was  the  first  tunnel  undertaking  in  which  compressed  air  was 
used  solely  with  the  object  of  supporting  the  ground  and  not  of  expelling  water. 

The  shields  were  of  the  ordinary  type. 

156 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  Siphon  de  Clichy1 

This  work,  which  consists  of  a  shaft  (receiving  one  of  the  main  outfall  sewers) 
on  the  Clichy  bank  of  the  River  Seine,  and  a  tunnel  from  it  under  the  river 
which  gradually  rises  to  join  the  open  masonry  drain  which  conducts  the  sewage 
of  Paris  to  the  sewage  farm  at  Acheres,  was  constructed  in  1892-4. 
|i?  It  is  of  interest  as  being  the  first  undertaking  in  France  in  which  the  Greathead 
shield  was  employed,  the  designs  for  the  shield  employed  having  been  supplied  by 
Mr.  Greathead  himself  ;  and  also  as  being  one  of  the  few  examples  of  compressed 
air  tunnelling  in  which  air  pressure  was  used  merely  to  hold  back  water  in  the 
fissures  of  otherwise  solid  and  easy  material  for  tunnelling.  It  is,  too,  important 
from  the  fact  that,  to  the  success  which  attended  the  employment  of  a  shield 
in  this,  and  in  the  similar  work  at  the  Pont  de  la  Concorde,  is  owing  the  extraordinary 
amount  of  tunnel  work  recently  carried  out  in  Paris  by  means  of,  in  the  first  place, 
shields  in  conjunction  with  masonry  linings,  and  secondly  by  roof  shields  under 
which  only  a  portion  of  the  permanent  tunnel  is  built  under  shield,  the  remainder 
being  constructed  in  timber  lengths  in  the  ordinary  way. 

The  shaft  forming  the  downward  limb  of  the  siphon  is  about  77  feet  deep, 
and  is  constructed  with  cast-iron  tubbing  nearly  10  feet  in  internal  diameter,  of 
the  usual  pattern. 

The  shaft  was  sunk  through  the  later  porous  beds  to  what  was  hoped  was 
fairly  watertight  material,  the  marls  of  the  limestone  beds,  and  the  tunnel  driven 
through  them  at  a  depth  of  60  feet  below  the  mean  water  level  of  the  river, 
and  with  a, minimum  of  29  feet  6  inches  between  the  roof  of  the  tunnel  and  the 
river  bed. 

The  sinking  of  the  shaft  was  carried  out  on  the  usual  lines.  The  excavation 
was  made  by,  in  the  first  place,  spade  work,  the  interior  of  the  shaft  being  kept 
dry  by  pumping.  A  grab  was  then  employed  and  finally  the  bottom  of  the  shaft, 
when  it  was  sunk  to  the  full  depth  required,  was  closed  by  a  diver  who  fitted  a 
domed  plate  as  an  invert.  'At  times  kentledge  to  the  amount  of  200  tons  was 
employed,  and  the  effect  of  this  was  increased  by  the  use  of  dynamite  cartridges 
which  loosened  the  more  compact  beds  passed  through. 

The  work  of  sinking  the  shaft  occupied  eight  months,  so  that  the  rate  of 
sinking  was,  on  the  average,  less  than  10  feet  each  month. 

The  lower  part  of  the  shaft  was  constructed  with  special  castings  to  allow 
of  opening  out  from  it  a  tunnel,  and  it  was  the  hope  of  the  engineers,  from  the 
results  of  the  borings,  that  at  that  level  the  material  was  sufficiently  watertight 
to  allow  of  an  easy  start  being  made  with  the  shield  work. 

But  the  experience  obtained  in  sinking  the  .shaft  showed  that  the  marl  and 
limestone  which  existed  at  that  level  were  so  much  fissured  that  water  came  through 
in  large  quantities. 

When  the  invert  of  the  shaft  had  been  sealed,  therefore,  an  airlock  was  fitted 
into  the  shaft,  and  the  special  castings  or  "  plug  "  removed  in  order  to  commence 
the  shield  chamber. 

The  airlock,  instead  of  being  fitted  to  the  top  of  the  shaft,  was  fixed  on  a  chimney 
or  tube  about  3  feet  6  inches  in  diameter  fixed  in  the  centre  of  the  shaft,  and  splayed 
out  to  meet  the  shaft  lining  immediately  above  the  tunnel  opening,  an  arrangement 
the  advantage  of  which  is  not  apparent. 

1  Legonez,  Emploi  du  Bouclier,  pp.  253-267. 
157 


TUNNEL    SHIELDS 

When  the  air  pressure  was  put  on,  the  "  plug  "  closing  the  tunnel  opening  was 
removed,  and  the  shaft  being  too  small  to  allow  of  the  erection  of  the  shield  within 
it,  it  was  necessary  to  build  a  small  brick  chamber  outside  the  shaft  in  which  to 
erect  the  shield.  This  brick  chamber  was  8  feet  10  inches  in  internal  diameter, 
and  12  feet  long.  It  was  closed  at  the  end  by  a  dome-shaped  brick  head  wall  which 
was  cut  away  when  the  shield,  to  introduce  which  the  airlock  had  to  be  removed 
and  replaced  after  the  shield's  erection,  was  ready  to  start.  Had  the  shield 
material  been  lowered  into  the  shaft  before  fixing  the  airlock,  for  which  there  would 
have  been  room  had  not  a  smaller  air  shaft  been  introduced  into  it,  this  removal 
and  re-erection  of  the  shield  would  not  have  been  necessary. 

The  shield  was  of  the  ordinary  Greathead  pattern  as  used  on  the  City  and 
South  London  Railway. 

Work  on  the  tunnel  was  commenced  with  the  vertical  airlock  in  use,  and  was 
so  carried  on  until  some  115  feet  of  tunnel  in  all  had  been  constructed,  when  a 
horizontal  airlock  was  fixed  in  the  tunnel,  and  the  vertical  one  removed  from  the 
shaft. 

The  horizontal  airlock  was  20  feet  long,  the  masonry  of  the  bulkhead  being 
24  feet  6  inches  from  front  to  back.  The  sides  of  the  lock  were  like  those  of  the 
City  and  South  London  Railway  without  any  lining  of  iron.  The  doors  were  of 
cast  iron  fitted  into  cast-iron  frames  built  into  the  brickwork. 

The  iron  lining  of  the  tunnel  is  7  feet  6|  inches  in  internal,  and  8  feet  2J  inches 
in  external  diameter.  Each  ring  is  1  foot  7£  inches  wide,  and  consists  of  five 
segments  and  one  key.  When  finished  the  tunnel  was  lined  with  concrete  to 
•the  inside  diameter  of  the  lining,  and  the  whole  surface  rendered  over  smooth. 

The  vertical  joints  are  packed  with  wood,  and  the  horizontal  ones,  which  as 
at  Glasgow  had  fillets  at  the  back,  with  cement  pointing. 

The  progress  of  the  tunnel  appears  to  have  been  uneventful,  and  as  regards 
speed,  fairly  uniform.  Save  in  open  sand,  or  where  a  bed  of  conglomerate  was 
encountered,  the  daily  advance  was  seldom  less  than  6  feet  6  inches  per  day,  and 
never  more  than  10  feet. 

The  maximum  air  pressure  employed  was  about  40  pounds  per  square  inch, 
and  it  is  to  be  regretted  that  more  exact  details  are  not  obtainable  as  to  the  char- 
acter and  extent  of  the  "  fissures  "  in  the  marl  and  limestone  beds  traversed. 

The  Siphon  de  la  Concorde,  forming  a  part  of  the  same  drainage  system  of  Paris, 
was  carried  out  a  year  or  two  later  by  the  same  contractor,  and  on  similar  lines. 

To  the  Greathead  shield  employed  on  this  work,  an  addition  was  made  of  a 
loose  curved  plate  or  "  hood,"  which  fitted  round  the  outside  of  the  skin,  and  could 
be  advanced  in  front  of  the  cutting  edge  bay  so  as  to  form  an  additional  protection 
for  the  miners.  This  plate  extended  from  the  top  of  the  shield  down  to  the  haunches 
on  either  side,  covering  one-third  of  the  circumference  of  the  shield,  and  when 
fully  extended  it  reached  about  1  foot  8  inches  in  advance  of  the  shield. 

This  was  the  first  occasion  on  which  a  shield  was  used  with  other  than  a  vertical 
face. 


158 


Chapter  VI 

THE  SHIELD  IN  WATER-BEARING  STRATA  (continued) 

THE  HUDSON  RIVER  TUNNEL — WORKS  IN  COMPRESSED  AIR  WITHOUT  SHIELD — A  BRICK 
TUNNEL  CONSTRUCTED  WITH  AN  ADVANCE  CASING  OF  IRON — FAILURE  OF  THE  TEMPORARY 
LINING  OF  THE  ENTRANCE — RECONSTRUCTION  OF  THE  ENTRANCE  BY  MEANS  OF  A 
CAISSON — THE  WORK  OF  TUNNELLING  BY  MEANS  OF  A  PILOT  HEADING — SUS- 
PENSION OF  THE  WORKS — RESUMPTION  OF  THE  WORKS  WITH  A  SHIELD  AND  IRON 
LINED  TUNNEL — THE  SHIELD  DESCRIBED — THE  MECHANICAL  ERECTOR — METHOD 
OF  WORKING — PROVISIONS  FOR  MEN  SUFFERING  FROM  COMPRESSED  AIR  SICKNESS — 
THE  ST.  CLAIR  RIVER  TUNNEL — DETAILS  OF  THE  SHIELD — METHOD  OF  WORKING — 
THE  MECHANICAL  ERECTOR — THE  BLACKWALL  TUNNEL — THE  CAISSONS  FORMING  THE 
SHAFTS — METHOD  OF  SINKING  THEM — LEVEL  OF  SUBAQUEOUS  TUNNEL  FIXED  WITH  IN- 
VERT 80  FEET  BELOW  WATER  LEVEL — DETAILS  OF  SHIELD — THE  FACE  SHUTTERS — 
THE  RAMS — THE  HYDRAULIC  ERECTORS — METHOD  OF  LOWERING  THE  SHIELD  FROM 
GROUND  LEVEL  TO  BOTTOM  OF  SHAFT — COMPRESSED  AIR  MACHINERY — THE  VERTICAL 
LOCKS  IN  THE  SHAFTS— SAFETY  SCREEN  IN  TUNNEL — METHODS  OF  DRIVING  SHIELD — 
INCIDENTS  OF  THE  WORK — CLAY  BLANKET  IN  RIVER  BED — METHOD  OF  WORKING  THE 
FACE  SHUTTERS  OF  THE  SHIELD — POLING  OF  THE  SHIELD  INVERT — CONDITIONS  OF  WORK 
IN  COMPRESSED  AIR — THE  EAST  RIVER  GAS  TUNNEL,  NEW  YORK — WORK  WITHOUT 
COMPRESSED  AIR — COMPRESSED  AIR  EMPLOYED — COMPRESSED  AIR  AND  SHIELD  USED 
TOGETHER — DETAILS  OF  THE  SHIELD 

The  Hudson  River  Tunnels  (1879) 

AS  mentioned  above,  the  same  year,  1879,  saw  the  application  of  the  com- 
pressed air  system  to  tunnel  work  in  Antwerp  and  New  York,  though,  as  at 
the  latter  place,  the  air  pressure  was  only  put  in  on  December  28  of  that  year,  the 
small  tunnel  or  adit  at  Antwerp  must  rank  as  the  first  tunnel  in  which  compressed 
air  was  actually  used. 

The  Hudson  River  tunnel  was  projected  with  the  object  of  connecting  the 
City  of  New  York  by  railway  with  the  termini  of  the  great  railways  from  Washing- 
ton and  the  South  in  Jersey  City.  They  were  designed  (for  the  idea  of  a  single  large 
tunnel  was  soon  abandoned — the  work  is  still  unfinished),  first,  as  two  single  line 
tunnels  of  elliptical  shape,  18  feet  high  and  16  feet  wide  inside  the  brickwork,  and 
are  constructed,  so  far  as  they  are  built  to  the  present  date,  entirely  in  a  very  soft 
mud  or  silt  of  an  argillaceous  character,  forming  the  river  bottom,  and  overlying 
for  the  most  part  sand  and  gravel.  This  material  is  water-logged  throughout.1 

The  total  length  of  the  tunnels  would  be,  when  completed,  nearly  6,000  feet 
from  shaft  to  shaft,  not  including  the  shore  tunnels  and  approaches,  and  the 
greatest  depth  of  the  tunnel  invert  below  mean  tide  level  is  about  100  feet. 

The  undertaking  is  a  particularly  interesting  one  to  engineers,  not  merely 
as  the  first  example  on  a  large  scale  of  tunnelling  by  means  of  compressed  air,  and 

1  Drinker's  Tunnelling,  p.  961.  Work  in  these  tunnels  has  recently  been  resumed  and 
carried  to  a  successful  conclusion  (Oct.  1905). 

159 


TUNNEL    SHIELDS 

unhappily  as  the  scene  of  one  of  the  most  unfortunate  mishaps  recorded  in  con- 
nexion with  the  system,  but  at  the  same  time  as  being  the  first  tunnel  where  Ander- 
sons' "  pilot  "  system  of  tunnelling  was  tried,  and  where,  later,  one  of  the  first  large 
shields  was  used  in  conjunction  with  compressed  air. 

Work  was  commenced  on  the  New  Jersey  side  of  the  Hudson  by  sinking  a 
brick  shaft  to  a  depth  of  60  feet,  in  which,  at  a  depth  of  23  feet  below  mean  tide 
level,  an  opening  was  made  and  an  airlock  A  (Fig.  99)  built  into  it,  from  which 
the  tunnel  could  be  started.  The  silt  there  exposed  was  of  so  fine  and  fluid  a 
character  that  immediately  it  was  exposed  to  the  air  it  commenced  to  run,  but 
this  tendency  was  stopped  when  the  air  pressure  was  put  on.  It  was  decided  to 
commence  the  break-up  for  the  tunnel  by  opening  out  and  timbering  the  roof,  but 
after  considerable  labour  the  idea  was  abandoned,  as  water  finding  its  way  down 
through  the  silt,  probably  through  the  blow-holes  made  by  the  compressed  air, 
practically  made  the  material  to  be  excavated  liquid  mud. 


FIG.  99.     HUDSON  TUNNEL,  NEW  YORK. 
The  New  Jersey  Shaft,  and  the  Tunnel  as  first  begun. 

It  was  then  resolved  to  try  to  drive  a  temporary  entrance  or  heading  from 
which  to  commence  the  construction  of  two  single  line  tunnels,  the  idea  of  one 
large  one  being  abandoned.  This  was  effected  by  erecting  outside  the  air-lock  two 
rings  B,  B,  formed  of  wrought-iron  plates  and  angles,  6  feet  4  inches  in  diameter  and 
4  feet  long  and  bolted  together ;  and  beyond  them  a  series  of  similar  rings  C,  C,  C, 
each  2  feet  6  inches  wide,  and  each  succeeding  one  increasing  about  1  foot  6  inches 
in  diameter,  until,  with  the  eleventh  ring,  the  full  diameter  of  the  permanent  tunnel 
was  reached. 

This  arrangement  of  rings  composed  of  segments  of  insufficient  strength  (for 
the  plates  were  only  \  inch  thick),  and  only  joined  to  each  other  at  the  crown, 
so  that  any  extra  pressure  on  a  ring  was  borne  by  that  ring  alone,  and  not  distri- 
buted in  any  way  on  the  adjacent  ones  as  in  an  ordinary  iron-lined  tunnel,  made 
a  very  unsatisfactory  lining,  and  its  instability  doubtless  disturbed  the  cohesion 
of  the  bad  material  with  which  it  was  surrounded,  and  aggravated  the  danger 
it  was  meant  to  resist. 

1 60 


THE    SHIELD    IN    WATER-BEARING    STRATA 

Such  as  it  was,  however,  it  served  for  some  time  as  a  means  of  access  to  the 
two  permanent  tunnels  which  were  then  commenced. 

The  lining  of  these  tunnels  was  constructed  in  the  following  manner.  The 
excavation,  the  air  pressure  being  maintained  sufficiently  high  (about  18  pounds  to 
the  square  inch)  to  keep  the  silt  stiff  and  dry  enough  to  permit  of  its  being  cut  in 
steps  or  benches,  the  one  above  the  other,  was  carried  forward  so  that  the  face 
of  the  working  sloped  upwards  and  forwards  at  about  1  to  1.  As  the  excavation 
advanced,  a  lining  of  wrought-iron  rings,  2  feet  6  inches  wide,  each  com- 
posed of  fourteen  plates,  was  put  in,  each  ring  being  commenced  from  the  top, 
so  that  of  the  five  or  six  rings  always  in  construction  at  the  same  time,  each 
ring  would  be  a  little  nearer  complete  than  the  one  in  front  of  it  (see  Fig.  99). 
These  plates  were  stiffened  with  angle  irons  which  indeed  formed  the  joints 
between  them.  The  six  top  plates  in  each  ring  were  3  feet  long,  the  remaining 
eight  being  about  4  feet  long  each.  As  these  rings  were  completed  they  were 
lined  with  bricks  in  cement.  (For  the  section  of  the  brick  tunnel,  see  Fig.  102.) 

It  will  be  seen  that,  in  this  method  of  tunnelling,  in  ground  such  as  was  passed 
through  at  the  Hudson  tunnel,  everything  depends  on  the  maintenance  of  the  re- 
quisite amount  of  air  pressure,  as  the  face  of  the  excavation  was  not  timbered  in 
any  way  nor  were  the  incomplete  iron  rings  stretched  at  the  bottom  by  timber. 
This  condition  of  equilibrium  between  the  air  inside  the  tunnel,  and  a  varying 
head  of  water  outside,  is  much  less  easy  to  maintain  in  actual  practice  than  to 
lay  down  as  a  desideratum  in  a  scheme  of  compressed  air  tunnelling,  and  it  is  some- 
what curious  that  among  the  numerous  mishaps  which  befel  the  undertaking  be- 
tween 1879  and  1889,  the  one  of  most  likely  occurrence,  namely  a  serious  "  blow  " 
at  the  top  of  the  face,  due  to  the  excess  of  pressure  in  the  tunnel  over  the  hydro- 
static head  at  the  level  of  the  crown  and  followed  by  complete  flooding  of  the  tunnel 
does  not  seem  to  have  happened. 

The  amount  of  air  pumped  into  the  tunnel  was  only  at  the  rate  of  125  cubic 
feet  of  air  per  man  per  hour,  about  one-thirtieth  of  the  amount  now  considered 
necessary  for  health. 

Another  feature  of  interest  in  the  work  in  the  more  northerly  of  the  two  tunnels, 
which  was  the  one  first  commenced,  was  the  method  of  getting  rid  of  the  excavated 
silt,  or  rather  of  so  much  of  it  as  was  not  left  in  the  invert  of  the  completed  tunnel, 
there  to  await  the  completion  of  the  tunnel  approaches. 

The  silt  was  not  carried  out  from  the  tunnel  through  the  airlock,  but  being 
mixed  with  about  25  per  cent,  of  its  own  bulk  of  water,  was  in  this  semi- 
fluid condition  blown  out  through  a  6  inch  pipe  D,  Fig.  99,  which  extended  from 
the  invert  at  the  working  face  where  its  mouth  was  inserted  in  the  mud  to  the 
shaft  in  the  open  air.  The  high  pressure  of  the  air  in  the  tunnel,  when  the  valve 
in  this  pipe  was  opened,  drove  the  mud  out. 

The  method  adopted  of  building  out  successive  incomplete  rings  of  iron  plates 
in  advance  of  the  permanent  brickwork  of  the  tunnel  might  have  answered  if  the 
extent  of  the  hood  or  canopy  so  made  over  the  working  face  had  been  kept  within 
reasonable  limits,  and  if  the  segments  forming  the  rings  had  been  more  massively 
constructed.  As  it  was,  in  the  northern  tunnel  the  temporary  rings  were  at  the 
last  advanced  as  much  as  55  feet  in  front  of  the  brick  lining,  and  owing  to  that 
fact,  and  their  too  light  construction,  they  became  distorted,  the  crown  settled 
down,  and  the  shape  of  the  tunnel  was  entirely  lost. 

When  work  was  suspended  in  this,  the  more  northern,  tunnel  some  six  months 
.  161  M 


TUNNEL    SHIELDS 

after  compressed  air  was  commenced  (June  23,  1880),  some  280  feet  of  tunnel  had 
been  constructed  ;  but  previous  to  this  date,  a  commencement  had  been  made 
with  the  southern  tunnel.  To  do  this  it  was  necessary  to  widen  out  the  temporary 
passage,  or  entrance,  of  the  north  tunnel,  an  operation  of  considerable  difficulty, 
particularly  in  view  of  the  peculiar  method  of  construction  of  the  iron  roof. 

A  bridle  plate  was  bolted  at  one  end  to  the  iron  rings  of  the  north  tunnel, 
and  at  the  other  end  to  the  corner  of  the  temporary  entrance  (the  6  feet  4  inch 
rings) ;  and  on  the  same  side  of  the  temporary  entrance  (the  southern)  a  side  exca- 
vation was  commenced  10  feet  in  length,  so  that  the  bridle  arch  could  be  put  in, 
the  plates  forming  it  being  propped  by  timbers  resting  on  foot  blocks.  This  exca-1* 
vation  was  made  deep  enough  to  clear  the  proposed  arch  and  the  central  pier  form- 
ing the  division  between  the  two  tunnels  got  in,  thus  forming  a  support  for  the  iron 
bridle  arch.  Pits  for  the  two  sidewalls  of  the  tunnel  arches  were  then  sunk,  and 
the  brickwork  got  in.  That  done,  the  invert  was  completed,  and  a  short  length 
of  iron  lining  of  the  southern  tunnel  built,  the  core  of  silt  being  left  in. 

The  brick  arch  was  then  carried  round  these  rings,  thus  completing  the  en- 
trance of  the  tunnels.  A  few  feet  of  tunnel  beyond  the  brick  eye  was  built,  and  the 
face  was  then  timbered,  and  the  serious  task  of  replacing  the  temporary  chamber, 
now  in  a  more  precarious  state  than  ever,  by  a  permanent  brick  one,  which  should 
enclose  the  entrances  to  both  the  single  line  tunnels  commenced. 

The  work  in  the  south  tunnel  was  carried  out  under  a  pressure  of  not  less 
than  21  pounds  per  square  inch,  which,  seeing  that  the  top  of  the  chamber  was 
only  some  25  feet  below  mean  water  level,  appears  rather  high,  and  likely  to  have 
put  some  strain  on  the  roof  plates. 

The  work  was  then  taken  in  hand  of  reconstructing  the  entrance  chamber, 
which,  as  already  described,  consisted  of  the  entrance  to  the  north  tunnel,  as  shown 
in  Fig.  99,  to  which  was  afterwards  added  the  side  excavation  necessary  for  the 
south  tunnel,  both  being  roofed  with  iron  plates,  and  closed  at  the  tunnel  end  by 
the  headwall  containing  two  eyes.  The  end  of  the  temporary  entrance  on  the  shaft 
side  had  no  permanent  headwall. 

The  work  of  making  a  permanent  brick  lining  to  the  chamber  was  commenced 
at  the  tunnel  end  of  the  temporary  one,  by  removing  one  by  one  the  existing  rings, 
and  substituting  for  them  similarly  constructed  rings  of  the  required  shape ;  that 
is  to  say,  the  new  arch  covered  the  area  occupied  by  two  previously,  or  in  other 
words,  the  temporary  entrance  to  the  single  line  tunnels  was  replaced  by  a  short 
length  of  double  line  tunnel  starting  at  the  working  shaft  and  ending  at  the  brick 
wall  already  built,  through  which  were  made  the  entrances  to  the  single  line  tunnels 
then  in  course  of  construction.  It  was  originally  intended  to  open  out  the  exca- 
vation so  as  to  put  in,  for  the  larger  tunnel  section,  iron  rings  similar  in  design 
to  those  already  described  as  being  used  in  the  north  tunnel,  and  to  follow  up  these 
immediately  with  the  permanent  brick  lining,  ring  by  ring,  until  the  chamber  had 
reached  far  enough  for  a  closing  wall  or  bulkhead  to  be  built  adjoining  the  working 
shaft.  But  in  actual  work  the  brickwork  lining  was  allowed  to  get  behindhand, 
so  that  by  the  time  the  roof  plates  had  been  put  in  as  far  as  the  wall  of  the  shaft, 
a  distance  of  some  30  feet,  only  the  brickwork  for  a  length  of  about  10  feet  was  in 
at  the  other  end  of  the  chamber,  and  that  only  for  the  height  of  4  feet  above 
the  invert.  The  work  had  reached  the  point  when  all  the  entrance  tunnel  had  been 
lined  with  iron  rings  almost  up  to  the  shaft,  and  the  miners  were  engaged  in  putting 
in  the  segments  of  the  rings  nearest  to  the  shaft  when  a  "  blow  "  occurred,  the 

162 


THE    SHIELD    IN    WATER-BEARING    STRATA 

tunnel  filled  with  water  so  rapidly  that  exit  through  the  lock  was  cut  off,  arid  twenty 
men  were  drowned. 

The  cause  was  undoubtedly  the  failure  of  the  roof  to  resist  the  air  pressure ;  or, 
in  other  words,  the  air  pressure  overcame  the  resistance  of  the  silt,  already  dis- 
turbed by  the  previous  mining  operations,  at  some  point  in  the  roof  of  the  chamber, 
probably  at  one  end  of  the  roof,1  a  large  escape  of  air  resulted,  the  pressure  in  the 
chamber  dropped,  and  at  once  the  roof,  deprived  of  the  sustaining  pressure, 
collapsed. 

There  can  be  no  doubt  that  the  disaster  was  caused  by  excavating  too  long 
a  length  in  front  of  the  permanent  lining,  as  was  done  with  the  same  ill-success  in 
the  north  tunnel. 

It  would  appear  that  entire  reliance  was  placed  on  the  sustaining  power  of  the 
air  pressure  in  the  working  chamber,  but  no  consideration  seems  to  have  been  given 
to  the  risk  run  in  working  in  this  manner  in  strata,  of  a  character  unsatisfactory 
to  commence  with,  which  had  been  completely  disintegrated  by  previous  operations. 


FIG.   100.     HUDSON  TUNNEL,  NEW  YORK. 
The  New  Jersey  Shaft,  and  method  of  construction  adopted  after  the  accident  in  July  1880. 


At  the  time  of  the  accident,  the  pressure  in  the  chamber  was  17  pounds,  which 
was  in  excess  of  the  amount  necessary  to  balance  the  hydrostatic  head,  and  con- 
sequently easily  capable,  given  a  place  in  the  roof  weakened  by  whatever  cause, 
of  blowing  out  through  the  superincumbent  material,  and  so  reducing  the  amount 
of  air  in  the  comparatively  small  pressure  chamber  that  practically  the  pressure 
would  drop  at  once  to  that  of  the  ordinary  atmosphere,  and  the  chamber  fill  with 
water. 

Had  the  permanent  lining  of  the  tunnel  followed  up  closely  the  excavation 
and  the  provisional  iron  plate  lining,  there  would  have  been  a  less  complete 
collapse.  As  it  was,  the  failure  of  one  or  two  rings  of  iron  probably  brought  down 
all  the  roof  of  the  chamber. 

After  an  unsuccessful  attempt  to  re-enter  the  collapsed  chamber  by  pumping 
out  the  water  which  had  at  the  moment  of  the  accident  filled  the  working  shaft 
above  the  level  of  the  airlock,  and  so  pass  through  the  lock,  the  construction  of  a 
caisson,  which  could  be  constructed  on  the  ground  level  above  and  sunk,  under 
air  pressure,  to  the  required  position,  was  resolved  on. 


1  The  position  of  the  blow,  as  indicated  by  dotted  lines  on  Fig. 
in  the  Scientific  American  of  August  7,  1880. 

163 


),  is  taken  from  a  sketch 


TUNNEL    SHIELDS 


The  caisson  was  made  of  timbers,  and  was  made  (see  Fig.  101)  l  rectangular 
in  form,  the  inside,  however,  being  shaped  to  the  radius  of  the  proposed  semi- 
circular arched  roof  of  the  chamber.  These  timber  arch  ribs  were  securely  braced 
to  the  outside  casing  of  the  caisson,  the  top  of  which  was  formed  of  a  double  floor 
of  heavy  timbers. 

The  sides  of  the  caisson  corresponding  in  position  to  the  end  walls  of  the  en- 
trance chamber  were  made  of  extra  thickness — the  inside  arched  roof  had  its  ends 
in  fact  closed  by  thick  bulkheads. 

Beneath  the  curved  arch  ribs  was  placed  4  inch  lagging  covered  with  lead 
and  asphalte  to  make  it  air-tight. 

The  whole  frame  when  complete  was  suspended  by  12  inch  screw  rods  3  inches 
in  diameter  and  terminating  in  wrought-iron  shoes  which  were  fixed  under  the  lower 

edges  of  the  caisson,  six  on  each 
side.  By  means  of  these  rods  the 
caisson  could  be  lowered  uni- 
formly, as  the  excavation  pro- 
gressed. On  the  top  of  the  caisson 
was  placed,  during  the  process  of 
sinking,  a  box  about  8  feet  in 
depth,  which  could  be  filled  with 
spoil  and  cast  metal  to  sink  the 
caisson  down  (not  shown  in  the 
figure). 

The  whole  was  made  large 
enough  to  contain  within  it  all  the 
semi-circular  upper  half  of  the 
proposed  chamber,  which  was  in 
length  nearly  30  feet,  and  in 
breadth  over  40  feet,  outside 
dimensions. 

It  was  entered  by  two  air- 
locks, the  one  for  men,  the  other 
for  the  supply  of  lime,  bricks, 
excavated  material,  etc. 

The  caisson  was  sunk,  with  a 
gradually  increasing  air  pressure, 
until  it  reached  the  position  re- 
quired (shown  in  Fig.  100),  when  the  wreck  of  the  old  temporary  roof  was  cleaned 
away,  and  the  bodies  of  the  drowned  miners  recovered. 

Communication  was  then  established  between  the  caisson  and  the  shaft  by 
repairing  the  damaged  horizontal  airlock,  and  driving  from  it,  under  air  pressure, 
a  small  heading,  4  feet  in  diameter,  to  the  caisson,  the  outside  of  which  was  3  or  4 
feet  distant  from  the  shaft,  and  the  whole  length  of  the  passage  so  made  lined  with 
iron  plates.  The  pressure  in  the  horizontal  lock  was  then  raised  to  equal  that  in  the 
caisson,  15  pounds,  and  the  inside  skin  of  the  caisson  consisting,  as  described  above, 
of  4  inch  lagging,  with  lead  and  asphalte  caulking,  was  cut  through,  and  communica- 
tion with  the  interior  of  the  caisson  established. 


FIG.   101.     HUDSON  TUNNEL,  NEW  YORK. 
Timber  caisson  used  for  constructing  Entrance  Chamber. 


1  The  Figure  101  is  copied  from  an  illustration  in  the  Scientific  American  of  Sept.  18,  1880. 

164 


THE    SHIELD    IN    WATER-BEARING    STRATA 

A  short  length  of  the  double  tunnel  invert  had  been  constructed  previous 
to  the  accident  of  July  21,  and  when  the  caisson  was  brought  to  rest  in  the  proper 
position  for  turning  the  arched  roof  of  the  tunnel  inside  it,  its  lower  edge  was  some 
14  feet  above  the  existing  brickwork  of  the  invert,  which  had  only  been  carried 
up  some  4  feet.  (The  ultimate  position  of  the  caisson  is  shown  in  dotted  lines  in 
Fig.  101.)  It  was  necessary,  therefore,  to  sink  small  pits  below  the  caisson  to  reach 
this  existing  masonry,  and  this  was  done  in  holes  4  feet  square,  which  when  sunk 
were  timbered,  and  on  the  outside  side  protected  with  iron  plates,  behind  which 
the  brick  walls  of  the  chamber  were  immediately  brought  up  to  the  underside  of 
the  caisson,  the  weight  of  which  these  walls  ultimately  took. 

By  sinking  successive  holes  of  this  kind,  the  length  of  double  tunnel  previously 
commenced  (it  was  8  feet  long  only)  was  finished,  and  then  by  short  lengths  the 
invert  was  gradually  extended,  and  the  corresponding  length  of  walls  and  arch 
constructed,  until  at  length  all  the  area  under  the  caisson  was  enclosed  in  brick- 
work, which  supported  the  caisson.  This  formed  the  brick  entrance  chamber  E  of 
Fig.  100. 

The  result  of  the  process  just  described  was  that  the  entrance  tunnel,  or  cham- 
ber, was  reconstructed  in  a  much  more  stable  and  satisfactory  condition  than  it 
had  ever  been  before,  and  with  improved  means  of  access  ;  the  tunnels  themselves, 
however,  were  still  full  of  water  and  silt,  and  were  known  to  be  so  damaged  that 
the  compressed  air  would  escape  without  expelling  the  water.  To  effect  an  entrance 
a  very  ingenious  plan  was  devised,  namely  the  utilization  of  the  fluidity  of 
the  silt  to  overcome  the  difficulty  caused  by  its  own  character.  It  was  resolved  to 
fill  up  the  tunnels,  full  as  they  were  of  water,  with  silt,  and  so  make  a  solid  mass 
in  which  mining  could  be  resumed. 

To  do  this,  a  small  hole  was  drilled  through  the  side  of  the  caisson  with  an 
auger  and  then  plugged  up,  and  this  was  repeated  until  the  opening  made  was  large 
enough  to  admit  a  pipe  2  inches  in  diameter,  which  was  driven  through  the  caisson 
into  the  tunnel.  The  end  of  the  pipe  within  the  caisson  was  bent  downwards,  a 
tap  being  also  fitted  to  it. 

The  downward  end  of  the  pipe  was  immersed  in  a  tank  of  silt  and  water  mixed 
to  a  fluid  mud  consistency,  and  the  tap  opened,  whereupon  the  air  pressure  in 
the  caisson  drove  the  diluted  silt  into  the  tunnel.  When  in  this  pipe  the  flow  of 
the  mud  ceased,  a  larger  one  was  substituted  and  again  a  larger,  until  ultimately 
a  6  inch  pipe  was  employed.  When  by  these  means  the  tunnels  were  filled  with 
silt,  the  caisson  was  cut  through,  and  a  small  tunnel  6  feet  in  diameter  constructed 
at  the  level  of  the  crown  of  the  brick  tunnel  already  completed. 

From  this  tunnel  as  from  an  ordinary  timbered  advance  heading  in  ordinary 
tunnel  work,  the  excavation  proceeded  sideways,  and  downwards,  until  the  whole 
of  the  tunnel  previously  excavated  was  cleared  out,  and  the  work  of  driving  for- 
ward recommenced  on  the  lines  previously  adopted. 

At  the  best,  the  methods  of  tunnelling  adopted  up  to  this  point  gave  a  maxi- 
mum daily  advance  of  4  feet. 

The  work  of  driving  the  tunnels,  which  had  been  interrupted  by  the  disaster  of 
July,  1880,  had  not  been  long  resumed  when  the  whole  system  of  tunnelling  was 
altered,  and  another  patented  system  known  as  Andersens'  Pilot  Tunnel  system 
was  adopted.  This  was  practically  the  adoption  of  an  iron-lined  tube  for  the 
timber  heading  of  ordinary  tunnel  work. 

A  small  central  iron  tunnel  or  heading  F  (see  Figs.  100  and  102)  was  driven 

165 


TUNNEL    SHIELDS 

some  3  or  4  yards  in  advance  of  the  iron  rings  forming  the  temporary  lining  of  the 
finished  tunnel.  This  iron  heading  consisted  of  rings  made  of  wrought-iron  seg- 
ments stiffened,  as  were  those  of  the  larger  tunnel,  with  angle  irons.  In  every  joint, 
both  between  those  of  the  rings,  and  of  the  segments  of  each  ring,  were  fixed  iron 
plates  H,  H,  H,  each  12  inches  wide,  and  arranged  so  as  to  form  cover  plates  for 
the  joints,  the  horizontal  ones  for  the  vertical  and  the  vertical  for  the  horizontal 
joints.  These  plates  projected  outside  of  the  cylindrical  heading,  and  greatly 
increased  its  stiffness. 

As,  by  the  removal  of  the  silt  round  each  successive  ring  of  the  pilot  heading, 
the  iron  lining  of  the  permanent  tunnel  was  put  in,  this  lining  was  supported  by 
timbers  J  J,  Fig.  102,  6  inches  by  4  inches,  stretched  against  the  longitudinal 


.     inn  i  i  i  i  i 

ui.12    o  « 


JOA. 


FIG.   102.     HUDSON  TUNNEL,  NEW  YORK. 

Sections  showing  comparative  areas  of  excavation  for  brick  lined,  and  iron-lined  tunnels  respectively ; 

and  Andersen's  Pilot  System. 

joint  plates  of  the  heading,  so  that,  so  to  speak,  the  pilot  tunnel  was  actually  serving 
as  a  kind  of  central  shaft,  from  which  radiated  spokes  which  supported  the  circle  of 
plates  round  it. 

As  rapidly  as  the  brick  lining  of  the  permanent  tunnel  advanced,  so  ling  by 
ring  the  stretchers  were  struck  and  the  rearmost  ring  of  the  pilot  tunnel 
was  taken  down  to  be  re-erected  in  the  front. 

By  this  method  it  was  found  that  the  tunnel  advanced  at  the  rate  of  5  feet  per 
day,  and  that  from  the  first  removal  of  silt  in  the  front  of  the  heading  to  the  com- 
plete enclosure  of  the  same  spot  in  the  finished  brick  tunnel  ten  days  were  occupied. 

After  some  experience  of  the  system  it  was  found  more  satisfactory  to  place 
the  pilot  tunnel  in  the  centre  of  the  larger  one,  as  indicated  in  the  dotted  lines 
in  Fig.  102,  instead  of  in  the  upper  part  of  the  tunnel. 

166 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  silt  excavated  was  taken  in  skips  drawn  by  a  steam  winding  engine  at 
K,  Fig.  100,  to  a  sump  near  the  tunnel  entrance,  whence  it  was  blown  out  through 
the  pipe  D,  D,  in  the  manner  already  described.  The  invert  of  the  completed 
tunnel  was  filled  with  silt  to  form  a  trackway,  as  shown  in  the  same  figure. 

After  the  tunnels  had  made  some  advance  the  locks  at  the  shaft  were  aban- 
doned and  new  locks  built  in  the  tunnel.  At  the  bulkheads  first  made  two  locks 
were  provided,  one  of  which  was  always  kept  with  the  inner  door  open,  to  serve  as 
a  safety  lock. 

The  works  described  were  all  carried  out  from  the  New  Jersey  side  of  the 
River  Hudson,  those  on  the  New  York  side  not  having  been  commenced  until  1881. 
There  the  engineers  commenced  operations  by  sinking,  instead  of  a  shaft,  a 
timber  caisson  similar  to  the  one  already  described  as  used  in  the  south  side  of  the 
river,  and  by  constructing,  immediately  the  caisson  was  sunk  to  the  required 
depth,  two  tunnels  in  brickwork  instead  of  one  single  arched  chamber  for  two 
tunneh  to  start  from. 

Much  difficulty  was  experienced  in  the  tunnelling  work  from  this  side,  due  to 
the  fact  that  the  material  passed  through  was  not  a  close  silt,  as  in  the  New  Jersey 
side,  but  mainly  gravel  and  sand.  It  was  found  necessary,  in  order  to  make  any 
advance,  to  pole  carefully  the  working  face  of  the  tunnel,  and  finally  to  protect  it 
with,  iron  plates  similar  to  those  used  for  the  temporary  tunnel  lining,  and  to  work 
very  small  areas  at  one  time. 

In  1882,  when  the  company  which  had  initiated  the  work  suspended  opera- 
tions, only  74  feet  of  the  northern  tunnel  had  been  driven  from  the  New  York 
side,  while  a  length  of  1,540  feet  was  completed  from  the  New  Jersey  end,  and  of 
the  southern  tunnel  which  was  practically  untouched  at  the  New  York  end,  600 
feet  was  built  from  the  New  Jersey  shaft. 

The  works,  so  far  as  they  had  then  gone,  proved,  if  they  proved  anything,  that 
compressed  air  alone  was  not  to  be  counted  on  to  overcome  the  difficulties  of 
tunnelling  in  water-bearing  strata,  at  any  rate  in  tunnels  the  vertical  height  of  which 
was  sufficient  to  produce  an  appreciable  difference  in  water  pressure  between  the 
crown  and  invert. 

Work  on  the  tunnels  was  suspended  from  1882  until  1889,  when  a  resumption 
was  made  under  English  management,  Messrs.  Pearson  and  Sons  being  the  con- 
tractors. 

For  some  time  the  method  of  working  with  Andersen's  pilot  tunnel  was  con- 
tinued, but  with  increasing  difficulty  as  the  tunnels  advanced  and  the  depth  below 
water  level  increased.  Work  on  the  New  York  side,  which  had  never  made  any 
great  progress  with  the  Haskin  system  of  compressed  air,  was  suspended  soon 
after  the  new  contractors  commenced  work,  and  after  some  500  feet  advance  had 
been  made  in  the  northern  tunnel  from  the  New  Jersey  end,  it  was  resolved  on  the 
advice  of  Sir  B.  Baker  and  Mr.  Greathead  to  substitute  for  the  masonry  tunnel  a 
cast-iron  one,  and  to  employ  a  shield  for  driving  it,  the  use  of  compressed  air  being 
of  course  continued. 

The  shield  (see  Fig.  103)  was  constructed  by  Sir  W.  Arrol,  and  its  working  was 
in  the  hands  of  Mr.  E.  W.  Moir.  It  was  10  feet  6  inches  long  and  19  feet  11  inches 
in  outside  diameter,  the  external  diameter  of  the  cast-iron  lining  to  the  tunnel 
being  19  feet  6  inches. 

In  general  arrangement  it  resembled  the  St.  Clair  shield,  with  two  important 
differences,  namely,  in  the  rigid  attachment  of  the  vertical  and  horizontal  girders 

167 


TUNNEL    SHIELDS 

to  the  main  diaphragm  of  the  shield,  and  in  the  provision  of  doors  in  the  diaphragm 
at  the  different  levels  of  the  working  platforms. 


1 

00 


J 

H       • 

fc  2 
- 


The  cylindrical  skin  was  formed  of  two  thicknesses  of  ^  inch  steel  plate,  in- 
ternal  stiffness  being   given  to  the    shield  by  a  plate  diaphragm    jj    inch    thick, 

1 68 


THE    SHIELD    IN    WATER-BEARING    STRATA 

pierced  by  nine  small  doors  ;  and  by  two  vertical  and  two  horizontal  girders  C,  C, 
D,  D.  immediately  in  front  of  the  diaphragm,  all  rivetted  together  at  their  respec- 
tive intersections  to  each  other,  and  at  their  ends  and  edges  to  the  cylinder  and 
to  the  diaphragm. 

The  diaphragm  was  fixed  4  feet  10  inches  from  the  tail  of  the  shield,  that 
distance  being  left  clear  for  the  erection  of  the  tunnel  lining.  The  circular  box 
girder  E,  which  consisted  of  a  series  of  cells  in  which  the  shield  rams  were  housed, 
extended  3  feet  4  inches  in  front  of  the  diaphragm  and  in  the  remaining  2  feet 
4  inches  of  the  length  of  the  shield  was  formed  the  cutting  edge,  the  inside  face 
of  which  consisted  of  a  conical  plate  F,  rivetted  in  front  to  the  skin  plates,  and 
behind  to  the  inner  circular  plate  of  the  box  girder. 

The  horizontal  girders  D,  D,  which  served  also  as  working  platforms,  were  also 
stiffened  in  front  by  a  somewhat  similar  arrangement  at  O,  G,  the  vertical  plate 
of  the  framing  forming  also  a  safety  curtain  for  the  miners  when  working  on  the 
platform.  There  were  originally  provided,  below  these  vertical  plates,  sliding 
shutters  shown  in  dotted  lines  in  Fig.  103,  which  were  arranged  to  work  on  angle 
iron  guides  H,H,  fixed  on  the  vertical  frames  and  on  the  inner  skin  of  the  box  girder, 
but  these  were  never  used,  the  material  proving  sufficiently  fluid  to  obviate  the 
necessity  for  any  work  in  front  of  the  main  diaphragm. 

The  shield  was  pushed  forward  by  sixteen  rams,  each  3  feet  4  inches  long,  and 
8  inches  in  internal  diameter,  and  each  capable  of  exerting  a  pressure  of  100  tons. 

With  the  shield  just  described  was  employed  the  hydraulic  erector,  shown  in 
Figs.  104,  105,  the  first  of  the  kind  ever  constructed. 

In  1875,  Mr.  Greathead  designed  and  patented  an  hydraulic  erector  for  use 
in  the  tunnel  under  the  River  Thames  at  Woolwich,  which,  however,  was  never 
actually  constructed.  In  its  general  features  the  one  used  at  the  Hudson  tunnel, 
which  was  designed  by  Mr.  E.  W.  Moir,  followed  Mr.  Greathead's  design. 

The  erecting  gear  was  carried  on  a  platform  consisting  of  three  plate  girders 
A,  A,  A,  bearing  on  and  secured  to  cast-iron  frames  S,  B,  which  were  carried  on 
wheels  bearing  on  rails  fixed  to  brackets  on  the  sides  of  the  tunnel. 

Directly  over  the  two  rearmost  girders  A,  A,  were  fixed  two  cast-iron  hydraulic 
cylinders  C,  C,  6  feet  in  length  and  6|  inches  in  diameter. 

The  rams  of  these  cylinders  had  at  their  ends  pulleys  D,D,n  inches  in  diameter. 

Between  the  two  girders  carrying  the  hydraulic  cylinders  was  fixed  a  drum  E, 
2  feet  3  inches  in  diameter,  this  drum  being  keyed  to  a  shaft,  7  inches  in  diameter, 
which  turned  in  journals  fixed  in  the  webs  of  the  three  girders,  and  carried  at  its 
forward  end,  and  at  right  angles  to  itself,  an  hydraulic  cylinder  with  forward  and 
reversing  action  framed  into  the  latticed  arm  F,  to  one  end  of  which  could  be 
secured  the  segments  of  the  tunnel  lining  by  means  of  the  lugs  H  H  specially  cast 
on  the  segments  for  that  purpose. 

The  stroke  of  the  ram  in  the  arm  F  was  3  feet  4  inches. 

On  each  of  the  cylinders  C,  C,  were  cast  lugs  J,  J,  to  which  were  secured  the  ends 
of  a  single  chain  which  passed  over  the  pulleys  D,  D,  and  round  the  drum  E. 

The  working  pressure  of  the  hydraulic  gear  was  about  1,000  pounds  per  square 
inch,  and  this  was  manipulated  by  the  levers  at  K. 

The  method  of  working  is  as  follows  : — When  a  cast-iron  ring  is  to  be 
erected,  each  segment  is  in  turn  brought  on  a  trolley  immediately  under  the 
arm  F,  which  is  made  to  point  to  it  by  working  one  or  other  of  the  cylinders  C,  C, 
so  as  to  make  the  drum  E  revolve  and  with  it  the  shaft  to  which  the  arm  F  is  fixed. 

169 


TUNNEL    SHIELDS 

When  the  arm  is  directed  properly,  it  is  made  to  extend  itself  toward  the  casting 
to  be  picked  up  by  working  the  cylinder  contained  in  it.  The  casting  being  secured 
to  the  end  of  the  arm,  the  cylinder  is  reversed,  and  as  the  arm  draws  back  it  lifts 
the  casting  with  it.  The  cylinders  C,  C,  are  again  put  into  action  and  the  arm  F 
is  swung  round  with  the  casting  at  the  end  of  it,  until  it  points  to  the  place  where 
the  segment  has  to  be  placed  ;  when  the  cylinder  in  F  is  made  to  push  the  arm 
forward  until  it  presses  the  segment  firmly  in  its  position,  where  it  holds  it  while 
the  miners  are  bolting  it  up  to  the  adjacent  ones. 

The  cast-iron  lining  of  the  tunnel  is  18  feet  in  internal,  and  19  feet  6  inches 
in  external  diameter,  each  ring  being  1  foot  6  inches  long. 


FIG.   104.     HUDSON  TUNNEL,  NEW  YORK. 
The  Hydraulic  Erector.     Elevation. 

When  the  cast-iron  lining  was  first  adopted  each  ring  was  composed  of  nine 
segments  and  one  key,  the  segments  being  of  graduated  size  and  thickness  from 
the  invert  to  the  crown  of  the  tunnel.  This  first  pattern  proved  too  weak,  and 
subsequently  the  lining  shown  in  Fig.  102  was  adopted. 

This  consisted  of  eleven  segments  and  one  key,  with  planed  joints  provided 
with  caulking  grooves.  The  thickness  of  the  webs  was  1J  inches. 

The  erection  of  the  shield  at  the  end  of  the  tunnel  already  constructed,  under 
an  air  pressure  of  30  pounds,  was  a  serious  operation,  and  the  necessity  of  using  a 
rivetting  furnace  in  such  an  atmosphere  had  a  very  serious  effect  on  the  health  of 
the  men,  but  when  once  the  shield  was  started,  the  work  went  on  very  regularly 
and  10  feet  advance  per  day  was  steadily  made. 

170 


THE    SHIELD    IX    WATER-BEARING    STRATA 

The  mud  or  silt  met  with  was  so  fluid  that,  even  against  the  great  air  pressure 
in  the  tunnel — 30  to  35  pounds — it  flowed  out  through  the  doors  in  the  diaphragm, 
and  when  the  shield  was  pushed  forward,  discharged  itself  into  the  tunnel  as  if 
pressed  out  of  a  mould. 

At  this  tunnel  there  was  used  also  an  expedient  frequently  tried  since,  where 
the  crown  of  a  tunnel  is  near  the  bed  of  the  river  above.  When  the  shield  was 
within  5  feet  of  the  river  bed  an  artificial  blanket  of  clay,  15  feet  thick,  was  laid 
above  the  soil  as  a  protection  while  the  tunnel  was  being  driven. 

The  effect  of  this  was  to  provide  on  the  bed  of  the  river,  and  covering  the  silty 
permeable  material  through  which  the  shield  was  passing,  a  solid  mass  of  clay, 
which  immediately  any  settlement  of  the  silt  took  place  sank  into  the  hole  thus 
formed,  and  automatically  caulked  it. 

In  the  year  1891,  however,  after  some  two  years'  work  by  Messrs.  Pearson, 
work  on  the  tunnels  was  suspended  owing  to  the  exhaustion  of  the  promoters'  funds, 
and  the  undertaking  has  remained  unfinished  up  to  the  present.1 


FIG.   105.     HUDSON  TUNNEL,  NEW  YORK. 
The  Hydraulic  Erector.     Plan. 


As  previously  remarked,  the  earlier  operations  in  the  undertaking,  while  they 
demonstrated  the  great  assistance  which  the  use  of  compressed  air  gives  in  affording 
actual  support  to  a  working  face,  so  long  as  that  face  is  not  fissured,  proved  clearly 
enough  that  compressed  air  was  not  a  substitute,  but  only  a  powerful  aid  to  the 
usual  protective  appliances  or  supports  used  in  tunnel  work.  When  used  with 
the  pilot  heading  of  Mr.  Andersen,  and  later  with  the  shield,  however,  work  was 
done  which  could  not  have  been  accomplished  by  these  appliances  unaided  by  com- 
pressed air,  and  it  is  precisely  the  succession  of  experiments  made  in  its  use  which 
make  the  Hudson  tunnel  so  especially  interesting  to  the  tunnel  engineer. 

The  use  of  compressed  air  for  the  first  time  on  a  large  scale  in  tunnel  work 
(the  second  period  of  active  prosecution  of  the  tunnel  works  was  contemporaneous 
with  the  work  on  the  St.  Clair  Tunnel)  afforded  also  an  opportunity  of  observa- 
tion, on  a  large  scale,  of  its  effects  on  the  health  of  the  men  employed.  Though  in 
the  St.  Louis  Bridge  Works,  and  at  the  East  River  Bridge,  careful  observations 
seem  to  have  been  made  of  the  pathological  effects  of  caisson  disease,  it  was  at  the 
Hudson  Tunnel  that,  for  the  first  time,  the  principal  causes  of  the  disease  were  dis- 

1  The  works  are  now  finished  (190.5). 
I7T 


TUNNEL    SHIELDS 

covered  and  a  practical  cure  found  for  many,  and  an  alleviation  for  all  of  the  cases 
of  sickness  among  the  men  due  to  compressed  air  work. 

Much  of  the  credit  for  the  practical  treatment  of  the  difficulty  must  be  given 
to  Mr.  E.  W.  Moir,  who  was  in  charge  of  the  tunnel  works  for  Messrs.  Pearson, 
when  operations  were  resumed  in  1889.  At  the  time  Mr.  Moir  arrived  to  take  over 
his  duties,  the  death  rate  due  to  caisson  sickness  in  the  Hudson  tunnel  was  at  the 
rate  of  25  per  cent,  of  the  men  employed.  Mr.  Moir,  by  employing  the  air  hospital, 
or  medical  lock,  suggested  by  Dr.  Smith  in  his  work  on  Compressed  Air  Sickness, 
at  once  reduced  the  excessive  mortality,  and  in  fifteen  months  only  two  deaths 
occurred  out  of  one  hundred  and  twenty  men  employed  in  the  tunnel,  or  say  T66 
per  cent,  as  compared  with  the  previous  figures  of  one  in  four. 

To  quote  Mr.  Moir's  own  description  1  of  the  arrangement  for  treating  the 
men  :— 

With  a  view  to  remedying  this  state  of  things  [that  is  the  serious  mortality]  an  air  com- 
partment like  a  boiler  was  made  in  which  the  men  could  be  treated  homeopathically,  or  re- 
immersed  in  compressed  air.  It  was  erected  near  the  top  of  the  shaft,  and  when  a  man  was 
overcome  or  paralysed,  as  I  have  seen  them  often,  completely  unconscious  and  unable  to  use 
their  limbs,  they  were  carried  into  the  compartment  and  the  air  pressure  raised  to  about  half 
or  two-thirds  of  that  in  which  they  had  been  working,  with  immediate  improvement.  The 
pressure  was  then  lowered  at  the  very  slow  rate  of  1  pound  per  minute,  or  even  less,  the  time 
allowed  for  equalization  being  from  twenty-five  to  thirty  minutes,  and  even  in  severe  cases  the 
men  went  away  quite  cured. 

It  was  on  this  work  that  the  serious  effect  of  even  a  small  additional  percentage 
of  impurity  in  the  compressed  air  on  the  health  of  the  men  was  first  noted,  and 
also  the  further  fact  that  a  sudden  increase  of  impurity,  that  is  of  carbonic  acid, 
in  the  air,  is  most  dangerous  when  it  synchronizes  with  an  increase  of  pressure. 


The  St.  Clair  River  Tunnel  (1888) 

This  tunnel,  which  is  known  also  as  the  Sarnia  Tunnel,  is  constructed  under  the 
St.  Clair  River,  which  carries  the  overflow  water  of  Lake  Huron  to  Lake  St.  Clair, 
and  forms  the  boundary  between  the  state  of  Michigan,  U.S.A.,  and  the  Province 
of  Ontario,  Canada,  and  was  built  to  connect  up  the  Railway  systems  of  the  two 
countries,  which  previously  were  only  in  communication  by  means  of  a  ferry. 

The  method  of  construction  adopted,  an  iron  tunnel,  built  by  means  of  a  shield, 
was  determined  by  the  successful  progress  of  the  City  and  South  London  Railway 
Works  under  Mr.  Greathead,  which  were  visited  and  examined  by  Sir  Henry  Tyler, 
at  that  time  President  of  the  Grand  Trunk  Railway  of  Canada. 

The  total  length  of  tunnel  constructed  under  shield  is  6,000  feet,  of  which 
2,300  feet  is  under  the  River  St.  Clair  (see  Fig.  106).  All  the  tunnel  was  con- 
structed with  the  aid  of  a  shield,  and  compressed  air  was  used  in  the  portion  im- 
mediately under  the  river. 

The  cast-iron  lining  is  21  feet  in  external  and  19  feet  10  inches  in  internal 
diameter.2 

The  material  through  which  the  tunnel  is  made  is  clay  of  a  very  soft  character, 
in  which  pockets  of  gravel  and  sand  were  met  with  from  time  to  time.  The  bed 
of  clay  was  only  38  feet  thick,  in  places,  the  material  above  it,  being  the  rim  bed,  con- 

1  Journal  of  the  Society  of  Arts,  May  15,  1896.  2  See  p.  64,  chap.  III. 

172 


THE    SHIELD    IN    WATER-BEARING    STRATA 


sisting  of  sand,  while  below  is  a  bed  of  shaly  rock,  containing  an  abundance  of 
natural  gas.1 

It  was  resolved  to  employ  two  shields  and  to  drive  the  tunnel  simultaneously 
from  the  Canadian  and  American  sides,  a  method  of  procedure  which  involved 
some  risk  in  making  the  junction  of  the  two  headings  under  the  river. 

The  shields,  weighing  80  tons  each  were  put  together  on  the  site  and  rolled 
down  inclined  planes  into  the  excavations  made  at  the  tunnel  mouths,  in  which 
timber  cradles  were  fixed  to  receive  them.  On  the  slope  of  the  excavation  longitu- 
dinal sleepers  of  12  inch  by  12  inch  timber  were  laid,  the  lowering  being  effected 
by  means  of  six  chains,  anchored  at  one  end,  passed  around  the  shield,  and  hitched 
at  the  other  at  the  top  of  the  slope.  The  shields  at  starting  were  so  arranged  that, 
after  rolling  down  the  slope,  they  arrived  on  the  cradles  prepared  for  them  exactly 
in  the  right  position,  as  regards  the  vertical,  for  commencing  work.  This  operation 
took  little  over  an  hour. 

The  portions  of  the  tunnel  on  either  side  of  the  St.  Clair  River  were  driven  with- 
out the  use  of  compressed  air,  1.994  feet  on  the  Canadian  side  and  1,716  feet  on 


'fl™ 

vf 


FIG.   100.     ST.  CLAIR  TUXXEL,  CANADA. 
Longitudinal  Section  of  Tunnel  under  the  St.  Clair  River. 

the  American  being  so  constructed.  Often  the  clay  was  found  to  be  so  soft,  that 
it  was  possible  to  force  the  shields  forward  into  it,  and  as  at  the  Hudson  tunnel 
the  material  was  frequently  not  excavated  in  front  of  the  shield,  but  flowed 
through  the  openings  in  the  diaphragm  as  the  shield  went  forward,  and  sometimes 
the  amount  of  clay  so  entering  the  tunnel  was  fifty  per  cent,  in  excess  of  the  cubical 
content  of  the  finished  work.  It  might  well  have  proved  cheaper  on  this  account 
to  have  used  compressed  air  throughout  the  tunnel  operations,  instead  of  only  in 
the  portion  actually  under  the  river. 

When  the  shields  arrived  at  the  river  banks,  bulkheads  with  airlocks  were 
constructed  in  the  tunnel,  and  an  air  pressure  varying  from  10  to  28  pounds  per 
square  inch  was  maintained. 

The  air  was  supplied  by  two  compressors  to  each  working  face,  but  £S  their 
cylinders  were  only  20  inches  in  diameter  with  24  inches  stroke,  and  as  the  pres- 
sure pipe  to  the  tunnel  was  only  6  inches  in  diameter  the  amount  of  free  air  sup- 
plied was  much  below  what  it  should  have  been,  and  it  is  not  surprising  to  learn 
that  there  were  many  cases  of  compressed  air  sickness,  and  that  three  cases  ended 


j_ 

1  Engineering  News,  Oct.  4,   1890,  p. 
173 


293. 


TUNNEL    SHIELDS 

fatally.  Doubtless  the  presence  of  natural  gas  in  the  shale  below  the  tunnel  did  not 
make  the  air  chamber  more  healthy.  A  slight  increase  in  the  air  pressure,  coin- 
cident with  an  increased  amount  of  natural  gas,  would  increase  the  number  of 
cases  of  sickness  at  once. 

At  the  commencement,  the  miners  were  in  the  habit  of  locking  out  by  means 
of  a  valve  4  inches  in  diameter,  but  this  was  found  too  rapid  in  action,  and  a 
1|  inch  one  was  substituted. 

The  locks  were  17  feet  long  and  6  feet  in  diameter,  and  in  each  bulkhead  a 
timber  lock  25  feet  long  and  10  inches  in  diameter  was  also  provided. 

As  is  usually  the  case  in  clay,  the  escape  of  air  was  small  except  when  pockets 
of  ballast  were  met  with,  and  the  air  blew  out  into  the  river.  The  amount  escaping 
was  kept  within  limits  by  plastering  the  ballast  and  sand  in  the  face  of  the  working 
over  with  clay,  and  only  working  a  small  part  of  the  face  at  a  time.  Another 
source  of  trouble  was  the  number  of  boulders  met  with  which  could  not  be  re- 
moved bodily,  nor  be  broken  up  by  explosions  for  fear  of  igniting  the  natural  gas 
in  the  tunnel,  and  therefore  had  to  be  split  by  mechanical  means. 

The  best  rate  of  progress  made  was  382  feet  at  the  American  face  in  the  month 
of  July,  1890,  or  say  12  feet  per  day.  From  the  commencement  of  the  shield  wrork 
to  the  journey  up  in  the  centre  of  the  river,  the  monthly  progress  at  each  face 
averaged  250  feet,  a  very  fair  rate  for  a  tunnel  of  its  size. 

The  two  shields  used  were  identical  in  pattern,  and  Mr.  Hobson  the  engineer, 
to  whom  their  design  is  due,  appears  to  have  followed,  so  far  as  he  has  followed 
any  one  model,  the  Beach  shield,  though,  as  stated  above,  the  employment  of 
the  shield  method  of  construction  was  suggested  by  the  satisfactory  progre  s 
then  being  made  with  the  City  and  South  London  Railway  under  Mr.  Great- 
head. 

Each  shield  was  15  feet  3  inches  in  length,  and  21  feet  6  inches  in  external 
diameter  (see  Figs.  107,  108). * 

The  cylinder  or  skin  was  made  of  plates  1  inch  thick,  and  was  made  in  four 
rings,  each  ring  consisting  of  twelve  plates  with  butt  joints.  The  front  ring  was 
3  feet  3  inches  long,  and  had  its  front  edge  chamfered  off  to  form  a  cutting  edge. 
The  other  three  were  4  feet  long.  The  last  of  these,  C,  formed  the  tail  of  the  shield, 
and  the  joints  of  the  plates  composing  it  were  covered  by  plate  covers  inside  and 
out,  |  inch  and  £  inch  thick  respectively.  All  the  other  joints,  longitudinal  and 
circumferential,  of  the  skin  plates  were  formed  by  angle  irons  rivetted  to  the  plates 
and  to  each  other  (see  Fig.  107). 

These  circumferential  joints  were  the  weak  part  of  the  design,  and  no  doubt 
were  responsible,  in  conjunction  with  the  fact  that  the  vertical  and  horizontal 
frames  of  the  front  of  the  shield  were  not  braced  to  the  diaphragm  behind,  for 
the  tendency  to  distortion  shown  when  working. 

At  the  tail  of  the  shield  an  attempt  was  made  to  reduce  the  escape  of  air  be- 
tween the  shield  and  the  last  ring  of  the  cast-iron  lining,  by  means  of  an  india  rubber 
collar  fixed  inside  the  skin. 

Two  angle  irons  D,  D,  3  inches  by  1^  inches,  and  having  a  space  between  them 
of  2|  inches,  were  rivetted  to  the  skin,  and  carried  right  round  (see  Fig.  107)  form- 
ing a  groove  into  which  rubber  packing  of  square  section  was  fitted,  and  held  in 
place  by  set  screws  working  through  one  of  the  angles.  It  was  hoped  that  this 

1  The  drawings  of  this  shield  are  reproduced  from  Engineering,  of  Nov.  14,  1890. 

174 


175 


TUNNEL    SHIELDS 

arrangement  would,  in  addition  to  reducing  the  opening  round  the  tunnel,  aid  also 
in  reducing  the  amount  of  friction  caused  by  either  the  tunnel  or  the  shield  being 
out  of  line,  and  so  bearing  heavily  on  one  side  or  the  other.  It  was  not  found 
possible,  however,  to  keep  the  rubber  from  tearing  out  of  its  channel,  and  a  metal 
ring  was  subsequently  substituted  for  the  rubber  with  satisfactory  results. 


FIG.   108.     ST.  CLAIR  TUNNEL,  CANADA. 
The  Shield.     Back  View,  showing  Erector. 


The  main  bulkhead  or  diaphragm  J  of  the  shield  consisted  of  plates  |  inch 
thick,  and  was  placed  4  feet  from  the  tail  of  the  shield,  and  at  the  junction  of  the 
two  rearmost  rings  of  the  cylindrical  skin,  and  to  the  angle  irons  forming  the  joint 
between  which  it  was  secured.  It  was  further  stiffened  by  three  vertical  and  seven 
horizontal  girders  K,K,K ,  made  of  plates  and  angles  (see  Fig.  107). 

This  bulkhead  was  closed  except  for  the  openings  through  which  the  rams 
for  driving  the  shield  passed,  and  also  for  two  openings  or  doors  L,  L,  at  its  lower  part 
which  could,  however,  if  required,  be  closed  by  sliding  doors  suspended  above  them. 

176 


THE    SHIELD    IN    WATER-BEARING    STRATA 

These  openings  were  6  feet  high  by  4  feet  6  inches  wide,  and  were  provided  with 
girders  in  which  the  sliding  doors  worked,  and  made  practically  air-tight  joints 
when  the  doors  were  down. 

The  position  of  these  doors  at  the  bottom  of  the  shield  does  not  appear  very 
satisfactory.  In  the  case  of  a  sudden  blow  in  the  face  it  is  likely  that  they  would 
be  choked  before  the  miners  working  in  the  upper  part  of  the  face  could  possibly 
escape.  At  the  Blackwall  and  Hudson  tunnels,  in  both  of  which  shields  with 
closed  diaphragms  were  used,  openings  were  provided  at  the  level  of  each  working 
platform  to  provide  for  this  contingency. 

Fortunately  the  necessity  of  closing  the  doors  never  arose  at  the  St.  Clair 
tunnel. 

To  compensate  for  the  weakening  of  the  diaphragm  by  the  openings  for  the 
rams,  an  extra  plate  M,  16  inches  wide  and  ^  inch  thick,  was  rivetted  all  round 
the  diaphragm,  which  was  further  strengthened  by  the  gussets  N,  N,  rivetted  to 
the  front  of  the  diaphragm  and  to  the  cylindrical  skin,  and  carrying  the  rams, 
twenty-four  in  number. 

The  tail  of  each  cylinder  was  fitted  in  a  ring  bolted  to  the  gussets,  and  the 
head  was  flanged  and  bore  against  the  main  bulkhead.  The  rings  are  shown  in 
the  section  G,  H  (Fig.  107),  and  the  flanged  heads  are  seen  clearly  in  the  detail 
drawing  in  Fig.  109. 

About  4  feet  in  front  of  the  bulkhead,  the  shield  was  stiffened  by  three  vertical 
and  two  horizontal  girders  O,  0,  0,  all  of  which  served  to  stiffen  the  shield,  and  the 
latter  of  which  formed  working  platforms.  It  will  be  seen  from  the  section  E,  F, 
in  Fig.  107-,  that  by  this  arrangement  an  open  space  4  feet  wide  was  provided  be- 
tween the  working  platforms  and  the  bulkhead,  thus  enabling  the  miners  to  cast 
the  spoil  from  the  face  down  into  the  invert  in  front  of  the  doors  in  the  bulkhead. 

A  peculiar  feature  in  the  shield  was  the  securing  of  the  bulkhead  to  the  vertical 
girders  by  straps  of  wrought  iron  P,  P,  P,  4  feet  9  inches  long,  7  inches  wide,  and 
|  inch  thick.  These  were  intended  to  support  the  bulkhead  or  diaphragm  in  the 
event  of  an  inrush  of  water  or  material,  when  of  course,  if  the  doors  in  it  were  closed, 
the  pressure  to  be  sustained  would  be  very  great. 

The  straps  in  question  were  useless  as  stiffeners  of  the  shield  :  had  the 
vertical  girders  been  carried  back  to  the  main  diaphragm  or  bulkhead,  the  shield 
would  have  been  much  stronger. 

It  was  found  in  actual  work  that  these  shields  required,  in  addition  to  the 
stiffening  provided  by  these  vertical  and  horizontal  girders,  considerable  timber 
framing  in  each  of  the  working  chambers  formed  by  the  girders. 

Probably  the  dividing  up  of  the  skin  of  the  shield  into  four  rings,  with  three 
cylindrical  joints,  and  the  coincidence  of  two  of  these  joints  with  the  rear  of  the 
bracing  girders  and  the  main  diaphragm  or  bulkhead,  respectively,  thus  leaving  the 
ring  between  them  entirely  without  support  beyond  that  given  by  the  gussets 
carrying  the  rams,  had  something  to  do  with  the  lack  of  stiffness  in  the  shield  ; 
the  insufficient  support  provided  for  the  cutting  edge  had  also  its  effect,  and  no 
doubt  the  vertical  and  horizontal  girders,  or  bracings  were  themselves  somewhat 
wanting  in  stiffness,  considering  the  material  the  shield  had  to  pass  through. 

The  rams  used  for  driving  the  shield  were  of  a  pattern  which  has  not  been 
imitated,  save  in  one  or  two  French  shields,  since.  The  main  rams  were  single 
acting,  but  provided  with  an  auxiliary  piston  for  drawing  back  the  piston  of  the 
ram  after  the  full  stroke  had  been  driven  (see  Fig.  109). 

177  N 


TUNNEL    SHIELDS 

The  main  cylinder  had  a  diameter  of  8  inches,  the  auxiliary  one  a  diameter 
of  2  inches,  and,  the  pressure  provided  being  about  1  ton  per  square  inch,  the  effec- 
tive thrust  of  each  was  45  and  4  tons  respectively. 

The  cylinders  were  of  cast  steel,  and  the  pistons  of  cast  iron,  the  head  of  the 
piston  of  the  main  cylinder  being  made  with  a  projection  on  the  outer  edge,  so  as 
to  bear  only  against  the  webs  of  the  tunnel  castings,  and  not  against  the  flanges. 
The  supply  of  water  to  the  cylinder  was  so  arranged  that  on  the  piston  reaching 
the  end  of  its  stroke,  the  pressure  was  automatically  lowered  so  as  to  avoid  breaking 
the  smaller  piston.  That  this  provision  was  necessary  will  be  seen  in  reference 
to  Fig.  109,  by  which  it  will  be  seen  that  without  some  such  precaution  the 
piston  could  have  blown  out  of  the  cylinder. 

The  small  cylinder  was,  so  to  speak,  always  under  pressure  ;  when  the  valve  of 
the  main  cylinder  was  open,  it,  the  small  cylinder,  was  forced  to  follow  the  greater 
pressure  ;  immediately  the  pressure  in  the  main  cylinder  was  cut  off,  the  smaller' 
one  asserted  its  power,  and  the  piston  of  the  larger  one  was  drawn'  back. 


Fig  10 

- 

1  . 

?'f 

$                  'a 

V    j 

i 

; 

FIG.   109.     ST.  CLAIB  TUNNEL,  CANADA. 
Hydraulic  Rams  of  the  Shield. 

This  arrangement  apparently  gave  satisfactory  results,  but  has  not  been 
followed  since  in  English  and  American  practice,  and  elsewhere  only  in  Paris, 
in  a  shield  on  the  Orleans  Railway  extension,  in  another  on  the  Metropolitan 
Railway,  and  in  a  third,  when  it  was  used  with  the  curious  twin  rams  of  the 
roof-shield  of  the  Meudon  Tunnel,  mainly  on  account  of  the  inconvenience 
caused  by  the  additional  length  occupied  by  the  auxiliary  cylinder,  which  amounted 
to  3  feet.  By  using  a  double  action  or  reversing  cylinder,  the  length  of  ram  re- 
quired is  only  that  necessary  for  driving  the  piston  in  one  direction,  plus  the  2 
or  3  inches  necessary  for  the  water  inlet  at  the  other  end.  The  only  advantage 
gained  by  the  use  of  a  smaller  auxiliary  cylinder  for  drawing  back  the  piston  is  that 
the  amount  of  water  required  for  the  operation  is  much  reduced. 

The  method  of  working  the  shields  does  not  present  any  special  features. 

Work  was  carried  on  day  and  night,  in  eight-hour  shifts,  and  it  is  said  that 
there  were  employed  in  each  working  face  seventy-five  men,  twenty-six  of  whom 
were  engaged  in  excavation. 

In  good  material — that  is  to  say,  when  the  clay  appeared  sufficiently  stiff— ex- 
cavation was  carried  on  in  front  of  the  shield  for  a  length  of  two  rings  of  the  cast- 

178 


THE    SHIELD    IN    WATER-BEARING    STRATA 


iron  lining  at  a  time  and  the  shield  then  pushed  forward,  but  usually  the  character 
of  the  clay  was  such  that  the  shield  followed  close  on  the  excavation  and 
no  unprotected  area  of  face  was  allowed.  Sometimes,  indeed,  as  mentioned  above, 
clay  of  so  soft  a  nature  was  met  with  that  it  actually  flowed  through  the  shield,  and, 
issuing  by  the  doors  in  the  diaphragm,  was  then  first  handled  by  the  miners  and 
loaded  into  skips.  This  was  frequently  the  case  when  nearing  the  end  of  the 
headings  which  had  been  driven  to  ascertain  the  character  of  the  material  to  be 
passed  through,  and  which,  having  collapsed,  were  filled  with  ballast,  and  other 
porous  material.  In  some  of  these  cases  the  air  pressure  required  to  keep  the 
tunnel  dry  was  as  much  as  35  pounds  to  the  square  inch. 

The  ground  in  front  of  the  shield  was 
always  tested  about  8  to  10  feet  ahead  by 
means  of  shell  augurs,  and  this  precaution 
proved  especially  useful  in  enabling  the 
men  to  detect  in  good  time  the  presence  of 
the  natural  gas  already  referred  to  as  pro- 
ducing an  ill  effect  on  the  miners'  health. 

It  was  found  that  the  best  tool  for 
getting  the  clay,  when,  as  was  sometimes 
the  case,  it  was  very  tough,  was  a  scraper 
formed  of  a  piece  of  hoop  iron  bent  to  a 
semicircle  of  about  6  inches  radius  and 
furnished  at  either  end  with  wooden 
handles,  much'  in  the  nature  of  a  joiner's 
spokeshave. 

The  cast-iron  tunnel  lining  is  described 
on  page  64,  and  for  its  erection  a  hand- 
worked erector  was  employed. 

It  was  an  arm  or  joist  E  mounted  (see 
Figs.  108  and  110)  on  a  pivot  S  fixed  on 
the  diaphragm  or  bulkhead  of  the  shield, 
or  rather  on  the  vertical  and  horizontal 
girders  which  stiffened  it. 

This  arm  was  made  to  rotate  on  the 
pivot  by  a  handle  inside  the  diaphragm  of 
the  shield  which  turned  the  wheel  T,  on  the 
shaft  of  which  was  an  endless  worm  U 
(behind  the  plate  bracing  shown  in  the  back 

elevation,  Fig.  108),  which  in  turn  revolved  the  wheel  V,  fixed  on  the  pivot  S  of 
the  erecting  arm.  By  this  arrangement  the  arm  could  be  directed  at  any  point 
in  the  circumference  of  the  tunnel.  The  arm  R,  which  is  shown  in  a  vertical 
position  in  Fig.  108,  and  details  of  which  are  given  in  Fig.  110,  terminated  in  a 
head  a  provided  with  four  arms  b,b,b,  between  each  pair  of  which  could  be  secured 
bolts  for  lifting  the  segments. 

A  segment  having  been  attached  to  the  arm,  it  was  lifted  clear  of  the  ground 
or  of  the  trolley  which  had  brought  it,  by  turning  the  handle  c,  which  actuated 
two  mitre  wheels,  d,  d,  which  turned  the  screw  e,  causing  the  head  a  to  draw  back 
towards  the  pivot  S,  bringing  of  course  the  segment  with  it. 

The  arm  was  then  swung  round  by  means  of  the  gearing  T7,  D,  V  (Fig.  108), 

179 


FIG.  110.     ST.  CLAIB  TUNNEL,  CANADA. 
Details  of  Erector  on  Shield. 


TUNNEL    SHIELDS 

and  directed  to  the  place  where  the  segment  was  to  be  placed,  when  the  head  a 
was  again  advanced  to  push  the  segment  into  its  place. 

To  facilitate  the  rotation  of  the  arm,  a  counter  balance  /  was  fixed  on  it. 

The  vertical  axis  of  the  arm  could  be  moved  nearer  to,  or  away  from,  the 
diaphragm  of  the  shield  by  applying  the  handle  c  to  the  screw  g  which  worked 
in  the  crosshead  at  the  end  of  the  shaft  S,  the  other  end  of  which  was  secured  to 
the  diaphragm. 

A  small  detail  in  the  working  arrangements  of  the  tunnel,  due  no  doubt  to 
the  fact  that  especial  accuracy  in  the  alignment  of  the  work  was  of  first  importance 
to  enable  the  two  shields  to  meet  exactly  in  line,  was  the  provision  of  a  special 
lock  or  pipe  1  foot  in  diameter  through  the  bulkhead  in  which  was  the  airlock,  for 
setting  out  the  centre  line.  For  this  purpose  this  pipe,  fitted  with  valves  at  either 
end,  had  near  its  extremities  fine  wire  "  cross  hairs  "  similar  to  those  in  a  theodolite, 
and,  like  'them,  capable  of  accurate  adjustment,  and  thus  serving  as  the  change 
points  whereby  the  line  outside  the  pressure  chamber  could  be  transferred  within. 


The  Blackwall  Tunnel 

This  tunnel  is  constructed  under  the  River  Thames  about  one  mile  below 
Greenwich,  and  connects  the  manufacturing  districts  of  Poplar  and  Blackwall, 
on  the  north  or  Middlesex  side  of  the  river,  with  East  Greenwich  on  the  Kent  or 
south  bank  (see  Fig.  111). 

The  river  at.  this  point  is  about  a  quarter  of  a  mile  in  width,  and  the  tunnel 
with  its  approaches  is  6,200  feet  in  length.  Of  this  total  length  1,735  feet  were  in 
the  open  approaches,  1,349  feet  in  cut  and  cover  tunnelling,  and  3,116  feet  in 
shield  and  compressed  air  work,  this  latter  portion  being  lined  with  cast-iron  seg- 
ments. 

The  external  diameter  of  the  cast-iron  lining  is  27  feet,2  and  the  internal  finished 
diameter  about  24  feet  8  inches,  affording  space  for  a  roadway  16  feet  in  width, 
with  a  headway  of  17  feet  8  inches,  and  two  footways  each  3  feet  in  width. 

It  is,  therefore,  much  the  largest  and  most  important  of  the  subaqueous 
tunnels  constructed  since  the  Thames  Tunnel  at  Rotherhithe  was  built  by  Brunei 
in  1825-42,  though  tunnels  of  30  feet  internal  diameter  have  been  built  in  the 
London  Clay,  and  it  is  only  in  the  past  year  (1904)  that  the  projected  road 
tunnel  also  under  the  Thames  at  Rotherhithe,  which  both  in  diameter  and  in 
length  will  exceed  it,  has  been  commenced. 

The  material  in  which  the  tunnel  was  driven  is  for  the  most  part  water-bearing, 
and  for  nearly  one-half  the  width  of  the  river  consisted  of  open  ballast,  the  water 
in  which  was  in  direct  communication  with  the  river  above,  and  as  from  considera- 
tions connected  with  the  men's  health  it  was  deemed  advisable  to  keep  the  tunnel 
within  80  feet  of  the  high-water  level  of  the  river,  the  shield  had  to  be  driven  in 
places  within  5  feet  of  the  bottom  of  the  river,  the  work  thus  having,  so  to  speak, 
to  be  carried  on  in  the  river  itself.  These  conditions  were  successfully  overcome, 
and  the  record  of  the  operations  is  an  important  part  of  the  history  of  suba- 
queous tunnelling. 

Fig.  Ill  shows  the  longitudinal  section  of  the  tunnel,  and  it  will  be  seen  that, 

1  Proc.  Inst.  C.E.  vol.  cxxx.     The  Blackwall  Tunnel,  by  David  Hay  and   Maurice  Fitzmaurice. 
This  paper  is  the  main  source  of  the  information  given  in  these  pages. 

2  For  details  of  the  cast-iron  lining  see  page  59. 

180 


THE    SHIELD    IN    WATER-BEARING    STRATA 


in  addition  to  the  open  approaches, 
access  is  obtained  to  the  tunnel  by 
means  of  shafts. 

The  open  approaches,  and  the  por- 
tions of  the  tunnel  built  by  cut  and 
cover  work,  were  carried  out  in  the  or- 
dinary manner,  and  the  details  of  their 
construction  do  not  come  within  the 
scope  of  this  work,  but  the  sinking  of 
two  of  the  shafts  and  the  working  of  the 
shield  was  conducted  entirely  in  com- 
pressed air. 

There  are  four  shafts  in  the  line  of 
the  tunnel  ;  Nos.  1  and  2  situated  on 
the  north  side,  and  Nos.  3  and  4  on 
the  south  side  of  the  river.  Their  posi- 
tions were  determined  by  the  horizontal 
or  vertical  changes  of  direction  of  the 
tunnel,  which  was  driven  straight  from 
shaft  to  shaft,  the  difficulty  of  driving 
the  shield  on  a  curve  and  the  trouble 
and  expense  of  special  castings  being 
thereby  avoided. 

The  avoidance  of  curves  in  the 
tunnel  was,  no  doubt  at  the  time,  a 
justifiable  precaution,  but,  given  suffi- 
cient ram  power,  the  shield  could  have 
been  driven  without  any  great  difficulty 
round  curves  with  a  radius  of  about 
800  feet. 

The  caissons  forming  the  shafts 
(see  Figs.  112  and  113)  are  48  feet  in 
internal,  and  58  feet  in  external  dia- 
meter, and  are  formed  of  two  skins, 
partly  of  steel,  and  partly  of  iron  rings. 
The  plates,  forming  the  rings,  generally 
4  feet  in  depth,  vary  in  thickness  be- 
tween |  inch  at  the  bottom  and  ^V  inch 
at  the  top  of  the  shafts  ;  they  are  stif- 
fened by  belts  of  angle-bars,  carried 
round  inside  at  every  lap-joint,  and  are 
braced  together  by  horizontal  and 
diagonal  angle-bars.  The  lower  portion 
is  constructed  of  steel  to  8  feet  6  inches 
above  the  cutting-edge,  which  is  formed 
by  bending  the  inner  skin  outwards  to 
meet  the  outer  skin,  and  is  stiffened  by 
a  band  1  inch  thick  and  2  feet  deep, 
running  round  the  outside,  and  by 

181 


owoa  vooa  VIOMI 


TUNNEL    SHIELDS 

vertical  plate  diaphragms  between  the  skins  (see  Fig.  114.)  The  inner  surface  of 
the  shaft  is  vertical,  the  inside  plates  of  each  successive  ring  being  set  at  such  an 
angle  as  to  bring  the  upper  edge  of  each  into  line  with  that  of  the  plate  below  ; 
the  outer  surface  has  a  batter  of  about  1  in  100  due  to  the  lap  of  the  plate  form- 


SJ&T.X"'  6  Col^P.C.  Concrete  ^'T^^\ 
'T-yn  .»*" %.;)\y.i>-i XV'^^'f ":  V^''  : '/. ^T^ 


^W\\v^\^\\\\v^x^ 


/o 


10 


30 


FIG.   112.     BLACKWALL  TUNNEL,  LONDON. 
Vertical  Section  of  Shaft  No.  2,  showing  position  of  Air  locks. 


ing   the   outside  of  each  ring  being    vertical,    and   ri vetted  inside   of  the  plate 
immediately  below  it. 

It  may  be  doubted  whether  a  batter  of  1  in  100  to  the  outside  of  such  caissons 
is  in  any  circumstances  advisable,  and  perhaps  some  of  the  difficulties  experienced 

182 


THE    SHIELD    IN    WATER-BEARING    STRATA 

in  sinking  the  caissons  at  Blackwall  were  due  to  the  considerable  difference  between 
the  top  and  bottom  diameters. 

The  Author's  own  experience  at  the  Greenwich  Tunnel  under  similar  conditions 
as  regards  the  nature  of  the  strata  passed  through,  was  that  caissons  with  parallel 
sides  were  entirely  satisfactory,  but  in  that  case  the  employment  of  a  second  air- 
tight floor  close  to  the  cutting  edge  of  the  caisson,  and  the  consequent  placing  of 
the  kentledge  used  below  the  centre  of  gravity  of  the  caisson,  was  also  an  impor- 
tant factor  in  keeping  the  caisson  plumb  during  the  process  of  lowering  it. 


FIG.   113.     BLACKWALL  TUNNEL,  LONDON. 
Section  of  Shaft  at  Tunnel  Opening. 

Mr.  E.  W.  Moir  has  suggested  1  that  a  better  arrangement  than  that  actually 
adopted  would  have  been  to  have  given  an  outside  batter  of  1  in  300,  or  to  have 
made  the  lower  part  of  the  caisson,  say  for  20  to  30  feet  above  the  cutting  edge 
with  the  outside  skin,  truly  cylindrical,  the  upper  part  being  tapered. 

Such  a  cylindrical  length  would  certainly  assist  in  keeping  the  caisson  steady 
when  going  down,  and,  by  being  limited  to  a  portion  only  of  the  shaft,  would 
avoid  the  risk,  always  possible  in  making  a  large  caisson  with  parallel  sides,  that 
the  upper  rings  in  the  process  of  assembling  and  rivetting,  by  becoming  distorted, 

1  Proc.  Inst.  C.E.,  vol.  cxxx.  p.   80. 
183 


TUNNEL    SHIELDS 

make  one  diameter  of  the  upper  part  of  the  caisson  larger  than  that  of 
the  cutting  edge.  The  space  between  the  skins,  5  feet  near  the  cutting  edge  and 
reduced  by  the  outside  batter  to  about  4  feet  at  the  top,  is  filled  with  6  to  1  Portland- 
cement  concrete. 

When  sunk  to  their  proper  depth  the  inverts  of  the  shafts  were  formed 
of  6  to  1  Portland-cement  concrete  about  1 1  feet  thick.  In  this  was  placed  a  skin 
or  plating  of  wrought  iron  with  the  object  of  making  the  invert  water-tight  (see 
Fig.  113),  but  it  was  in  no  sense  a  structural  support  to  the  concrete,  and  still  less 
a  bracing  to  the  shaft. 


CLttajctirruenL  ±or 
watertight  ±Loon 


FIG.   114.     BLACKWALL  TUNNEL,  LONDON. 
Detail  of  Cutting  Edge  of  Shaft. 


During  the  process  of  sinking  the  caissons,  brackets  formed  of  plates  and  angle 
irons  were  attached  to  the  inside  plates  of  the  cutting  edge  to  serve  as  bearings 
from  which  the  caisson  could  be  held  by  timber  packings  from  foot-blocks  on  the 
bottom  of  the  excavation. 

In  each  shaft  two  openings  were  provided  through  which,  when  the  shaft 
was  sunk  to  its  full  depth,  the  tunnel  could  be  constructed.  During  the  sinking 
of  the  caissons  these  openings  were  closed  by  a  "  plug  "  or  frame-work  b,  Fig.  113, 
consisting  of  girders,  vertical  and  horizontal,  having  bolted  to  them  plates  or 
shutters  c,  c,  which,  when  the  plug  was  in  position,  formed  a  part  of  the  external 

184 


THE    SHIELD    IN    WATER-BEARING    STRATA 

plating  of  the  shaft.  The  girders  were  bolted  to  the  sides  of  the  opening,  and  the 
whole  was  made  water-tight  by  wood  packing  and  caulking,  and  in  the  circum- 
ferential joint  by  current  caulking.  The  caisson  itself  is  strengthened  round  these 
openings  by  extra  plate  diaphragms  and  bracings,  and  the  circular  plates  forming 
the  opening  itself  are  extended  into  the  caisson  so  as  to  form  a  hood  d,  the  end 
of  which  is  vertical,  and  which  serves  as  a  circular  stiffening  plate  round  the  opening, 
to  which  numerous  gussets  can  be  fixed,  as  at  e,  e. 

Provision  was  1  made  for  attaching  air-tight  floors  to  the  inner  skin  of  caissons, 
above  the  level  of  the  tunnel  openings,  for  use  either  in  sinking  the  shafts  or  con- 
structing the  water-tight  floors  should  compressed  air  be  found  necessary,  or  when 
it  was  required  in  order  to  connect  the  tunnels  with  the  shaft.  The  floor  consisted 
of  f-inch  buckled  plates  resting  on  girders  /,  /,  18  inches  deep  ;  above  these  4-foot 
girders  g,  g,  were  placed  at  right  angles,  the  whole  being  surmounted  by  two  girders 
h,  h,  12  feet  deep.  The  ends  of  the  girders  and  the  buckled  plates  were  attached  to 
the  inner  skin  of  caisson,  which,  in  the  case  of  the  12-foot  girders,  was  strengthened 
by  a  ^-inch  doubling-plate  to  take  up  the  shear.  Stiff  iron  diaphragms  or  bulk- 
heads were  also  built  between  the  skins  in  the  line  of  the  two  main  girders.  When 
the  air  pressure  exceeded  20  pounds  per  square  inch  above  atmospheric  pressure, 
the  upward  thrust  on  the  floor  was  relieved  by  10  feet  or  14  feet  of  water-ballast. 

The  use  of  one  floor  only  for  sinking  the  shaft,  and  subsequently  for  starting 
the  tunnel  work — or,  if  the  shaft  be  one  towards  which  the  shield  is  driven,  for 
completing  it — has  the  advantage  of  economy  in  first  cost  over  the  use  of  two  floors, 
one  near  the  cutting  edge  for  sinking  the  shaft,  and  the  other  above  the  tunnel 
opening,  as  in  the  case  shown  in  Figs.  152  and  153,  for  the  subsequent  work,  but  for 
convenience  of  working,  and  easy  control  of  the  caisson,  during  sinking,  the  provision 
of  a  second  or  lower  floor  is  to  be  recommended.  The  fact  that  by  its  use,  the  centre 
of  gravity  of  the  kentledge  employed  is  kept  below  the  centre  of  gravity  of  the 
caisson  itself,  makes  for  stability,  and  of  course,  the  lower  the  floor  is  placed,  the 
greater  is  the  amount  of  kentledge  which  can  be  loaded  on  it.  The  cost  of  fitting 
such  a  floor  would  have  been,  in  the  case  of  shafts  of  the  size  of  those  at  Blackwall, 
about  £2,000,  against  which,  however,  must  be  set  the  cost  of  the  water-tight 
plating  subsequently  used  in  the  concrete  in  the  invert,  which  with  a  second  lower 
floor  would  be  unnecessary. 

A  further  cost,  however,  is  that  of  the  extra  depth  of  shaft  necessary  with  the 
second  floor,  and  some  idea  of  this  may  be  obtained  by  comparing  the  Blackwall 
caisson  in  Fig.  112  with  the  Greenwich  one  in  Fig.  152.  In  the  latter  case  the 
extra  depth  required  for  the  lower  floor  involved  an  expenditure  of  £1,500  extra 
in  the  shaft,  and  the  floor  itself  cost  an  equal  amount.  But  when  all  these  con- 
siderations are  allowed  for,  the  lower  floor  is  to  be  recommended  in  all  cases  where 
compressed  air  is  likely  to  be  required. 

At  Blackwall  water  was  used  for  kentledge,  and  there,  as  elsewhere,  the  ease 
with  which  it  can  be  put  in  or  taken  out  of  the  caisson,  and  its  utility  when  in,  in 
limiting  the  escape  of  air  through  the  air-tight  floor,  made  it  very  convenient  in 
application.  Its  use,  however,  has  one  drawback,  and  that  is,  that  in  case  of  a 
sudden  settlement  of  the  caisson  on  one  side,  the  rapid  adjustment  of  the  water  to 
the  new  levels  tends  to  increase  the  load  on  the  side  where  the  settlement  has 
taken  place,  and  so  to  aggravate  the  fault. 

1  Originally  a  lower  second  floor  at  the  cutting  edge  was  proposed,  but  cut  out  at  the 
wish  of  the  contractors. 

185 


TUNNEL    SHIELDS 

Pipes  were  provided  in  the  caissons  by  which  water  under  pressure  could  be 
forced  outside  them  near  the  level  of  the  cutting  edge  with  the  object  of  "  lubri- 
cating "  the  outside  surface  and  so  facilitating  the  sinking,  but  the  results  obtained 
were  only  meagre. 

The  shafts  on  the  south  side  of  the  river  were  sunk  by  pumping  down  the 
water  and  removing  the  excavation  from  the  inside  in  the  ordinary  way.  The 
weight  of  caisson  and  concrete  filling  was  generally  sufficient  to  overcome  the  skin 
friction,  until  about  20  feet  from  the  bottom,  when  more  weight  had  to  be  tem- 
porarily added.  The  sinking  of  No.  4  shaft  was  accomplished  without  difficulty, 
the  ground  being  good  and  almost  cleared  of  water  by  the  pumps  for  the  cut-and- 
cover  portion  and  the  open  approach  about  200  yards  distant ;  and,  as  very  little 
water  was  found  below  the  London  Clay,  it  was  possible  to  construct  the  water-tight 
floor  in  the  open.  Considerable  trouble  was,  however,  experienced  in  dealing  with 
shaft  No.  3  at  the  river  bank.  The  ground  for  about  25  feet  of  the  lower  part 
of  the  shaft  was  found  to  be  composed  of  fine  sand,  and  numerous  "  blows  "  were 
caused  by  the  head  of  water  in  the  ballast  above  (charged  afresh  by  every  tide), 
washing  in  large  quantities  of  gravel  and  sand.  The  ground  was  of  such  a  heavy 
clinging  nature  that  the  caisson  was  often  stationary  for  several  days,  although  the 
cutting-edge  was  free  and  the  structure  was  heavily  loaded  ;  and  it  was  not  until 
a  blow  occurred,  the  movement  of  the  ballast  decreasing  the  skin  friction,  that 
further  downward  progress  could  be  made.  The  caisson  was  at  one  time  14|  inches 
out  of  level,  and  various  expedients  were  tried  to  induce  blows  on  the  high  side  in 
order  to  correct  it.  The  best  results  were  obtained  by  striking  heavily  on  the  inside 
of  the  shaft  with  a  baulk  of  timber  swung  horizontally  from  a  girder  above.  Con- 
siderable subsidence  of  the  surface  was  caused  by  the  sand  and  gravel  entering 
the  shaft ;  the  wharf  wall  completely  disappeared,  leaving  the  river  free  to  surround 
the  caisson  at  every  tide  and  to  find  its  way  through  the  numerous  cracks  in  the 
London  Clay  to  the  heading  of  the  advancing  tunnel.  Some  damage  was  also 
caused  to  some  buildings  40  yards  distant. 

As  all  the  weight  had  been  placed  on  the  skins  for  which  room  could  be  found, 
the  girders  of  the  air-tight  floor  were  fixed  and  loaded  ;  better  progress  was  then 
made,  and  when  the  top  water  was  cut  off  no  difficulty  was  encountered  in  dealing 
with  the  springs  in  the  sand  below  the  London  Clay  ;  the  floor  was  therefore  con- 
structed, as  in  the  case  of  shaft  No.  4,  without  the  aid  of  compressed  air.  In  both 
floors  the  suction-pipes  of  the  pump  were  carried  through  the  iron  water-tight  floor 
near  the  bottom  of  the  concrete,  and,  after  ascertaining  that  no  sand  was  being 
drawn  out  with  the  water,  pumping  was  continued  until  the  concrete  was  thoroughly 
set,  when  the  valves,  which  had  been  provided  in  the  suction-pipes  near  the  top 
of  the  concrete,  were  closed.  The  friction  on  the  outside  skin  for  the  last  20  feet  in 
the  case  of  No.  3  amounted  to  about  5- 6  cwt.  per  square  foot,  the  cutting-edge 
being  entirely  free.  Little  or  no  reduction  of  friction  resulted  from  the  outside 
batter  in  this  case,  as  there  is  little  doubt  that  the  ground  closed  in  almost  in- 
stantaneously as  the  caisson  descended.  As  it  is  sunk  upon  private  property  this 
shaft  is  covered  by  a  dome,  the  inner  skin  of  the  caisson  being  set  back  about  34 
feet  b:low  the  surface  to  form  a  springer.  A  chimney  70  feet  high  is  built  on  the 
centre  of  the  dome  for  ventilation,  and  to  prevent  fumes  from  a  neighbouring  tar 
distillery  above  from  entering  the  tunnel. 

On  the  north  side  of  river  No.  2  shaft  was  sunk,  behind  a  new  river  wall,  Fig. 
Ill,  which  had  been  built  to  reclaim  part  of  the  foreshore.  From  borings  taken 

186 


THE    SHIELD    IN    WATER-BEARING    STRATA 

round  the  site  it  was  ascertained  that  the  whole  of  the  ground  to  be  penetrated 
consisted  of  ballast  ;  and  to  avoid  the  difficulty  and  delay  experienced  on  the  south 
side,  it  was  decided  to  employ  a  grab  for  the  excavation,  allowing  the  water  to 
remain  inside.  The  grab  was  worked  by  a  10-ton  crane  supported  on  girders  inside 
the  shaft  ;  and,  as  the  material  was  removed,  the  caisson  was  forced  down  by 
weighting  it  heavily,  in  addition  to  the  concrete,  with  tunnel  castings  distributed 
round  the  top  and  on  special  brackets  attached  to  the  inner  skin.  The  total  weight 
of  the  caisson  and  sinking-load  amounted  to  nearly  4,000  tons,  the  skin  friction  being 
therefore  about  4J  cwt.  per  square  foot  of  outside  surface.  Trouble  was  found 
at  first  in  working  the  grab  on  account  of  the  twisting  of  the  long  wire  ropes  ;  flat 
ropes  were  therefore  substituted,  and  no  further  difficulty  arose,  the  rate  of  sinking 
averaging  1  foot  in  twenty-four  hours. 

As  it  was  not  practicable  to  construct  the  floor  without  the  aid  of  compressed 
air,  the  water  was  pumped  down,  after  the  caisson  had  reached  its  correct  level, 
sufficiently  low  to  admit  of  the  air-tight  floor  being  fixed.  Divers  were,  however, 
previously  employed  to  fix  stools  on  the  inner  skin  of  the  cutting  edge,  and  packings 
were  inserted  beneath  to  prevent  further  sinking  of  the  shaft  in  consequence  of  the 
lowering  of  the  water.  Clay  bags  were  also  placed  round  the  cutting-edge  to 
prevent  any  ballast  running  in.  When  the  air-tight  floor  was  completed,  the  water 
remaining  inside  was  partly  blown  and  partly  pumped  out,  and  the  work  of  con- 
structing the  floor  proceeded.  An  air  pressure  of  35  pounds  to  37  pounds  per 
square  inch,  the  highest  employed  on  any  part  of  the  work,  was  necessary  to  dry 
the  ground  at  the  cutting-edge.  Although  this  shaft  was  sunk  within  6  feet  of 
the  river-wall  previously  referred  to,  no  crack  or  settlement  of  any  kind  was 
caused. 

The  first  28  feet  of  No.  1  shaft  were  sunk  in  the  same  manner  as  Nos.  3  and  4, 
but  on  reaching  this  depth,  large  quantities  of  water  were  encountered,  and  the 
grab  system  was  again  used.  This  method  was  continued  until  the  cutting-edge 
had  passed  through  the  ballast  and  sufficiently  far  into  the  clay  below  to  cut  off 
the  top  water.  The  water  inside  was  then  pumped  out  and  manual  labour  was 
again  employed.  It  was  found,  however,  that  the  caisson  was  not  sinking  uni- 
formly, owing,  no  doubt,  to  some  part  of  the  ground  offering  more  resistance  to 
the  cutting-edge  than  others,  and  additional  weight  had,  therefore,  to  be  placed  on 
the  high  side.  The  continual  rocking  of  the  caisson,  due  to  its  falling  out  of  level 
and  being  corrected  by  weighting,  no  doubt  broke  up  the  ground  to  a  certain  extent, 
and,  in  addition,  the  shaft  would  probably  carry  down  some  ballast  with  it  in  sinking. 
The  effect  was  a  serious  blow  under  the  cutting-edge,  and  the  shaft  was  again 
filled  with  water.  As  the  air-tight  floor  was  not  in  position,  it  was  decided  to 
provide  a  sump  outside  in  order  to  lower  the  head  of  water,  that  inside  the  shaft 
being  meanwhile  allowed  to  rise.  The  sump  was  sunk  to  a  depth  of  34  feet,  and 
the  head  of  water  lowered  about  20  feet.  The  water  in  the  shaft  having  been 
pumped  out,  preparations  were  made  for  constructing  the  floor  as  in  shafts 
Nos.  3  and  4,  but  on  clearing  out  the  bottom,  the  ground  was  found  to  be  too  soft 
to  support  the  weight  of  the  caisson,  which  began  to  sink,  although  the  packings 
were  in  position  under  the  stools.  The  construction  of  the  floor  in  the  open  was 
abandoned,  and,  on  the  completion  of  the  air-tight  floor  (which  had  meanwhile 
been  proceeded  with),  it  was  carried  out  under  an  air  pressure  of  28  pounds  per 
square  inch. 

Although  in  uniform  strata,  such  as  that  found  in  No.  2  shaft,  the  grab  may 

187 


TUNNEL    SHIELDS 

be  employed  with  good  results,  the  control  of  the  structure  is  imperfect,  and  it  is 
extremely  difficult  to  maintain  it  vertical  in  sinking.  The  same  remark  may  apply 
to  sinking  without  compressed  air  in  variable  material  where  water  is  encountered, 


FIG.   115.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Longitudinal  Section. 

because  bad  ground  immediately  becomes  softened  by  the  water  and  the  caisson 
falls  out  of  the  vertical  before  measures  can  be  taken  to  prevent  it.  With  com- 
pressed air,  however,  the  difference  between  what  is  known  as.  "  good  "  and  "  bad  " 
ground  may  disappear  altogether,  and  it  may  even  be  that  what  is  bad  ground  in 

188 


UNIVERSITY 


THE    SHIELD    IN    WATER-BEARING    STRATA 

the  open  is  good  working  material  in  compressed  air  ;  as,  for  example,  wet  sand, 
which  in  the  open  air  is  perhaps  the  most  treacherous  of  all  materials  in  mining  work, 
but  when  partly  dried  by  compressed  air,  is  the  easiest  of  all  kinds  of  excavation. 

The  caissons,  when  the 

i 

work  of  construction  was 
complete,  were  finished  in- 
ternally with  a  lining  of 
brickwork,  the  invert  being 
composed  'as  shown  in  Fig. 
112,  of  about  11  feet  of 
cement  concrete  above  and 
below  the  wrought  iron 
water-tight  floor. 

In  two  shafts,  Nos.  1 
and  4,  circular  staircases  are 
placed  ;  in  shaft  No.  2,  the 
necessary  pumps  for  the 
drainage  of  the  tunnel  are 
fixed,  and  shaft  No.  3,  as 
stated  above,  was,  on  the 
completion  of  the  tunnel, 
arched  over,  and  only  a  brick 
ventilating  chimney  marks 
its  place  on  the  surface. 

The  actual  work  of 
driving  the  tunnel  under 
shield,  and  with  compressed 
air  as  distinct  from  cut  and 
cover  work,  was  commenced 
at  No.  4  shaft  in  the  south 
or  Kent  side  of  the  river. 
The  determination  of  the 
depth  at  which  the  tunnel 
could  be  safely  and  economi- 
cally driven  was  matter  for 
very  serious  consideration, 
and  the  position  of  the  tun- 
nel was  ultimately  fixed 
partly  by  engineering  and 
partly  by  traffic  considera- 
tions. 

It  was  not  possible  on 
the  north  side  of  the  river 
to  extend  the  approach  be- 
yond the  East  India  Dock 
Road,  nor  was  there  room, 
owing  to  docks  and  other 
properties,  to  improve  the  gradient  between  this  road  and  the  river  by  "  develop- 
ment," or  lengthening  of  the  approach.  It  followed  therefore  that  the  gradient 

189 


FIG.  116.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Half  Cross  Section. 


TUNNEL    SHIELDS 

on  that  side  accepted  as  necessary  for  the  accommodation  of  the  traffic  deter- 
mined the  depth  of  the  tunnel  below  the  river. 

It  was  found  that  with  a  ruling  gradient  of  about  1  in  35,  the  tunnel,  for  which 
27  feet  had  been  fixed  as  the  minimum  external  diameter  capable  of  accommodating 
wheeled  traffic,  would  have  to  be  constructed  with  its  invert  about  80  feet  below 
highwater  mark,  and  with  the  crown  about  5  feet,  at  the  worst  place,  below  the 
bed  of  the  river. 

This  depth  of  80  feet  below  high  water  was  satisfactory  from  another  point  of 
view,  namely,  the  capacity  of  miners  to  work  under  the  air  pressure  corresponding 
to  that  depth.  It  was  judged  by  the  engineers,  though  subsequent  experience 
proved  that  there  was  considerable  difference  between  the  theoretic  and  actual 
air  pressure  required,  that  the  pressure  of  35  pounds,  corresponding  to  the  80  feet 
of  depth  decided  on,  was  the  greatest  air  pressure  per  square  inch  which,  on  general 
grounds  of  the  men's  health,  cost  of  work,  etc.,  could  be  considered  economically  safe. 

The  nature  of  the  ground  to  be  passed  through  was  known  as  well  as  is  usual 
in  such  cases  by  means  of  borings  and  dredgings  in  the  river,  and  in  preparing 
plans  for  the  shield  the  Contractor's  Engineer,  Mr.  E.  W.  Moir,  to  whom  the  design 
is  chiefly  due,  had  before  him  the  fact  that  he  had  to  design  a  shield  which  would 
have  to  cope  in  turn  with  clay,  with  water-logged  sand  and  ballast,  as  well  as 
with  the  various  hard  strata  known  as  the  Woolwich  beds. 

This  facility  of  working  in  whatever  kind  of  material  was  met  with,  was  the 
first  condition  of  a  successful  machine. 

It  was  also,  bearing  in  mind  the  variable  nature  of  material  anticipated,  neces- 
sary to  make  the  frame  of  the  shield  of  sufficient  strength  and  with  stout  bracing 
to  resist  unequal  pressure  on  its  outside  cylindrical  skin,  as  well  as  in  front,  without 
interfering  unduly  with  the  miners'  access  to  the  working  face.  How  strong  the 
shield  had  to  be  is  best  shown  by  the  fact  that  the  total  thrust  of  the  shield  rams 
amounted  to  over  5,000  tons,  and  that  this  pressure  was  actually  employed  in  driving 
the  shield  through  the  water-bearing  ballast. 

This  was  done  by  making  the  cylindrical  casing  of  the  shield  a  circular  box 
girder  for  the  greater  part  of  its  length,  and  by  placing  within  the  double  cylin- 
der so  formed  vertical  and  horizontal  girders,  which  latter  also  served  as  working 
platforms  strongly  framed  together,  as  well  as  two  vertical  diaphragms  each  com- 
pletely closing,  except  for  the  working  doors,  the  whole  area  of  the  shield.  These 
diaphragms  served  also  to  protect  the  tunnel  behind  from  any  irruption  of  water 
from  the  face,  and  also  when  the  shield  was  designed,  were  intended  to  accommodate 
air  locks,  it  being  at  one  time  intended  to  work  the  shield  with  a  different  air 
pressure  in  the  front  to  that  in  the  tunnel  itself,  and  in  the  upper  compartments  of 
the  shield  as  compared  with  that  on  the  invert.  This,  however,  was  never  actually 
done,  the  practical  difficulties  in  the  way  of  maintaining  differential  pressures  in 
compartments  in  close  proximity  to  each  other,  and  both  communicating  with  the 
same  face  of  material,  being  found  insuperable. 

But  though  these  two  diaphragms  were  useful  only  in  strengthening  the  shield, 
the  removable  diaphragm  formed  of  numerous  independent  plates  or  shutters, 
provided  at  the  front  end  of  the  shield,  by  which,  when  required,  practically  the 
whole  face  of  the  shield  could  be  closed,  proved  of  the  greatest  possible  utility,  and 
the  success  of  the  work  may  be  said  to  be  due  as  much  to  facility  of  working  on 
bad  ground  which  the  provision  of  the  shutters  allowed,  as  to  any  other  feature  in 
the  machine. 

190 


THE    SHIELD    IN    WATER-BEARING    STRATA 

In  the  Hudson  River  Tunnel  a  similar  arrangement  of  shutters  was  proposed, 
and  provision  made  for  them  in  the  shield  ;  they  were,  however,  never  actually 
used,  the  silty  material  met  with  there  not  calling  for  their  employment. 

The  curtain  plates  provided  at  the  face  under  the  main  horizontal  girders, 
were  also  similar  to  those  of  the  Hudson  River  shield. 


*RONT          ELEVATION.  BACK  ELEVATION. 

FIG.   117.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    half  front  and  back  elevations. 

The  details  of  the  shield  are  shown  in  Figs.  115  to  120,  122,  123,  and  124. 

Its  total  length  was  19  feet  6  inches  and  its  outside  diameter  27  feet  8  inches. 
The  skin  consisted  of  four  thicknesses  of  f  inch  steel  plates.  The  plates  were 
made  the  same  length  as  the  shield,  and  in  each  layer  they  broke  joint  with  the 

191 


TUNNEL    SHIELDS 

plates  above  and  below  them.  There  were  twenty-eight  plates  in  each  layer,  the 
width  of  each  plate  being  therefore  about  3  feet. 

The  rivets  in  the  skin  plates  were  |  inch  in  diameter  and  3|  inches  pitch, 
all  having  countersunk  heads  on  the  outside. 

The  front  ends  of  these  plates  were  bevelled  off  to  form  a  cutting  edge,1  and 
at  the  rear  end  of  the  shield  a  steel  strip,  3  inches  wide,  was  rivetted  on  the  inside 
so  as  to  form  a  ring  which  partially  closed  the  space  between  the  skin  and  the  cast- 
iron  tunnel  lining,  and  so  reduced  the  escape  of  air,  as  well  as,  in  some  measure, 


K 


H 


"E 


T 


D 


TJ 


-R 


FTG.   118.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    sectional  Plan. 


the  friction  between  the  two  surfaces  when  the  shield  was  being  driven  forward, 
particularly  during  a  change  of  direction. 

The  actual  cutting  edge  was  formed  of  the  skin  of  the  shield  only,  but  about 
12  inches  from  the  front  it  was  supported  all  round  by  inclined  plates  A  A,  which 
sloped  outward  from  the  inner  flange  of  the  circular  box  girder  B,  B.  This  box 
girder,  the  rear  end  of  which  was  6  feet  7  inches  from  the  cutting  edge,  consisted  of 
a  ring  of  |  inch  plates  .connected  to  the  skin  of  the  shield  by  plates  C,  C,  also  of  |  inch 


1  This  cutting  edge  was  subsequently  strengthened  by  the  addition  of  a 
the  outside,  see  page  209. 

192 


inch  strip  on 


THE    SHIELD   IN   WATER-BEARING   STRATA 

plate,  which  extended  completely  round  the  shield.     The  inside  diameter  of  this 
box  girder  was  25  feet  3  inches  in  diameter,  and  its  length  4  feet  3  inches. 

Behind  this  was  another  similar  but  deeper  circular  box  girder  D,  D,  which 
contained  the  rams  for  forcing  the  shield  forward,  to  accommodate  which  the  two 
rearmost  of  the  gusset  plates  in  the  box  had  holes  cut  in  them,  the  rams  bearing 
on  the  front  gusset  E.  The  internal  diameter  of  the  box  girder  D  was  24  feet, 
and  its  length  6  feet,  the  plates  being  f  inch  thick. 

The  remainder  of  the  skin  forming  the  tail  of  the  shield,  6  feet  8  inches  in 
length,  was  unsupported.  It  will  be  seen,  however,  that  of  the  total  length,  19 
feet  6  inches,  of  the  shield,  as  much  as  10  feet  3  inches,  or  if  the  sloping  plates  A,  A, 
be  included,  1 2  feet  7  inches  is  contained  in  a  double  cylinder  formed  of  a  circular 
box  girder,  divided  longitudinally  and  transversely  into  cells  by  the  gussets  C,  C, 
and  E,  E.  There  are,  it  is  to  be  noted,  no  cross  joints  in  the  skin  of  the  shield,  nor, 
except  at  the  junction  of  the  boxes  B  and  D,  in  the  inside  frame. 

The  framing  inside  this  double  cylinder  was  of  equal  solidity.  About  6  feet 
8  inches  from  the  rear  ends  of  the  shield  was  a  solid  diaphragm  F,  composed  of 
plates  f  inch  thick,  and  3  feet  in  front  of  this  again  a  second  similar  diaphragm 
G.  These  completely  divided  the  working  face  of  the  shield  from  the  tunnel,  the 
only  apertures  in  them  being  the  air  locks  H,  H,  and  the  shoots  J,  J,  which  could 
all  be  closed  and  worked  as  airlocks  in  case  it  was  necessary  to  work  with  a  higher 
air  pressure  in  the  face  of  the  shield  than  in  the  tunnel. 

It  will  be  noticed  that  while  there  are  twelve  shoots  J,  J,  or  one  to  each  working 
compartment  in  the  shield,  there  are  only  four  large  airlocks,  two  in  the  first 
platform,  and  two  on  the  uppermost.  Access  to  the  invert  and  to  the  second  floor 
was  provided,  however,  by  manholes  at  K  and  L  (Fig.  116)  respectively,  and  after 
the  idea  of  working  with  differential  pressures  in  the  shield  and  in  the  tunnel  was 
abandoned,  some  of  the  plates  at  the  bottom  of  the  diaphragms  were  removed 
and  direct  access  thereby  given  to  the  invert  at  the  face  from  the  tunnel.1 

In  front  of  these  diaphragms  the  shield  was  stiffened  by  three  horizontal 
girders  M ,  M,  M,  and  three  vertical  ones  N,  N,  N,  securely  rivetted  to  the  circular 
box  girders  and  to  each  other,  this  forming  a  rigid  frame  9  feet  3  inches  long. 

The  horizontal  girders  were  further  stiffened  by  the  frames  0,  0,  0,  which 
formed  a  kind  of  curtain  under  each. 

To  these  horizontal  girders,  as  well  as  to  the  crown  of  the  shield,  were  attached 
curtain  plates  P,  P,  P,  which  were  intended  to  serve  as  a  protection  to  the  men 
working  at  the  face  in  a  higher  pressure  than  that  in  the  tunnel. 

The  sliding  shutters  for  closing  the  face  of  the  shield  were,  when  in  position, 
in  a  line  with  the  rear  end  of  the  frames  0,  0,  0,  their  ends  sliding  on  the  planed 
guides  R,  R,  R  (Fig.  116),  fixed  on  the  vertical  girders  W,  W,  W.  Fig.  119  shows 
their  arrangement  in  the  face  of  the  shield,  and  Fig.  120  the  details  of  the  construc- 
tion of  one  of  those  in  the  central  compartment.  From  Fig.  119  it  will  be  seen 
that  the  area  of  the  face  through  which  material  could  pass  when  the  shield  was 
being  pushed  forward  amounted  to  380  square  feet,  the  total  area  of  the  shield 
front  being  600  square  feet. 

There  were  in  all  thirty  of  these  shutters,  of  varying  length,  and  generally 
1  foot  6  inches  in  depth.  Each  consisted  of  a  f-inch  iron  plate,  stiffened  at  the 
edges  by  heavy  angles,  6  inches  by  4  inches  by  f  inch,  and  sliding  on  guides  fixed 

1  Drainpipes,  shown  in  dotted  lines  in  Fig.  115,  were  also  provided  for  carrying  away 
water  from  each  floor. 

193  o 


FIG.   119.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Sliding  Shutters  of  the  Face. 


FIG.   120.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    details  of  Sliding  Shutters  of  the  Face. 


194 


THE    SHIELD    IN    WATER-BEARING    STRATA 

to  the  sides  of  the  compartment.  The  shutters  were  controlled  by  long  screws, 
fixed  to  their  ends  and  extending  through  bearings  on  the  side  of  the  compart- 
ment. There  were  two  slots  machined  the  whole  length  of  each  screw,  and  two 
keys  in  each  bearing  ran  in  these  slots,  thus  providing  a  means  of  graduating  the 
rate  of  movement  of  the  shutters  if  necessary.  Two  nuts  running  on  the  screw, 
one  on  each  side  of  the  bracket,  made  it  possible  either  to  shove  forward  or  draw 
back  the  shutter. 


T 


FIG.   121.     BLACKWALL  TUNNEL,  LONDON. 
"  Walking  Joint  "  for  Hydraulic  Mains. 


The  number  of  hydraulic  rams  originally  provided  for  advancing  the  shield 
was  twenty-eight  ;  they  were  8  inches  in  diameter  and  had  a  stroke  of  4  feet ;  but 
while  driving  through  the  wet  sand  and  ballast  under  the  river  this  number  of  rams 
was  found  insufficient,  and  was  therefore  increased  by  six  other  rams,  10  inches  in 
diameter,  but  with  a  shorter  stroke.  The  maximum  water-pressure  used  was 
about  2|  tons  to  the  square  inch,  or  a  total,  when  all  the  rams  were  employed,  of 
5,165  tons,  making  no  allowance  for  the  friction  of  the  rams.  The  pressure  was 

195 


TUNNEL    SHIELDS 

obtained  direct  from  pumps  on  the  surface  and  the  water  was  conveyed  to  the 
shield  by  means  of  1J  inch  steel  pipe,  to  which,  immediately  behind  the  shield,  a 
"  walking  joint  "  was  fitted.  The  type  of  joint  employed  here,  and  in  similar 
positions  on  the  hydraulic  erectors,  etc.,  is  shown  in  Fig.  121. 

The  pressure  was  distributed  to  the  shield  rams  through  nests  of  valves  on 
the  shield  in  the  usual  manner,  save  that  the  rams  were  arranged  to  work  in  pairs, 
that  is  there  were  only  half  the  number  of  controlling  valves  which  would  have 
been  required  if  each  ram  had  separate  pushing  and  drawing  valves. 

The  rams  were  attached  to  the  shield  by  bolts  through  the  collar  of  the  cylin- 
der and  the  rearmost  gusset  E  of  the  circular  box  girder  D,  the  collar  bearing  solid 
on  this  gusset.  The  forward,  or  butt  end  of  the  cylinder  bore  on  the  foremost  of 
the  three  gussets  E,  E,  E,  a  packing  block  being  fitted  to  the  latter  to  give  the  end 
a  good  bearing  (see  Fig.  115). 

The  design  of  the  rams,  and  particularly  the  arrangement  for  the  drawback, 
whereby  a  defective  leather  in  the  glands  could  be  removed  without  the  necessity 
of  unstripping  the  ram,  is  due  to  Mr.  Moir,  and  is  shown  in  Fig.  122.1 

The  cylinder  and  piston  are  turned  out  of  wrought  steel  blocks. 

The  cylinder,  which  is  4  feet  9  inches  over  all,  is  1 1  inches  in  external,  and  8 
inches  in  internal  diameter,  and  the  piston  with  a  4-foot  stroke  terminates  in  a 
head  bevelled  in  the  usual  manner  to  ensure  that  its  thrust  should  bear  on  the  skin, 
and  not  on  the  flanges  of  the  tunnel  castings. 

This  piston  was  bored  with  a  3-inch  diameter  hole  to  within  3^  inches  of  the 
end,  the  hole  being  closed  at  the  front  end  by  a  brass  plug  in  the  ram  head,  thus 
making  a  second  cylinder  within  the  piston.  In  it  was  a  steel  tube,  2  inches  in 
external  diameter,  which  was  screwed  into  the  end  of  the  cylinder,  making  a  water- 
tight fit  with  the  piston  where  it  passed  through  its  base. 

On  this  2-inch  tube  were  fitted  at  either  end  U-shaped  leathers,  and  at 
its  front  end  a  small  cross  hole  connected  the  inside  of  the  tube  with  the  cylinder 
in  the  piston.  At  the  other  end  of  the  tube  it  connected  with  the  main  hydraulic 
high  pressure  supply  by  means  of  the  pipe  and  cross  hole  in  the  end  of  the 
cylinder. 

It  will  be  seen  that  pressure  was  supplied  to  the  main  cylinder  at  one  end 
only  by  the  pipe  and  cross  hole  D. 

The  pressure  was  maintained  continuously  in  the  small  centre  piston,  so  that, 
immediately  water  was  allowed  to  escape  from  the  main  cylinder  after  using,  the 
constant  pressure  in  the  smaller  one  forced  the  main  piston  back  again  automati- 
cally. 

It  will  be  seen,  if  repairs  to  the  glands  were  necessary,  that  by  removing  the 
brass  plug  in  the  ram  head,  access  could  be  obtained  to  the  phosphor  bronze  nuts 
on  the  end  of  the  2-inch  tube.  These  removed,  the  leathers  could  be  removed  from 
and  replaced  in  the  piston  without  removing  the  entire  cylinder  from  the  shield. 

The  smaller  auxiliary  rams  added  after  the  shield  had  started  were  of  the 
same  type,  Fig.  123.  As  they  could  not  be  fixed  permanently  to  the  shield,  owing 
to  lack  of  space,  they  were  employed  in  the  invert  and  bore  against  the  rearmost 
gusset  E  of  the  box  girder  D,  an  extension  block,  Fig.  124,  being  used  for  the  latter 
part  of  the  push  for  each  ring  of  the  tunnel. 

Two  hydraulic  erectors  for  placing  in  position  the  cast-iron  segments  of  the 

1  See  Minute?  of  Proc.  InsL  C.E.,  vol.  oxvii.  p.  27- 
196 


THE    SHIELD    IN    WATER-BEARING    STRATA 


tunnel  lining  were  provided.  They  were  fixed  on  the  main  diaphragm  F  (Figs. 
115,  116)  of  the  shield,  one  on  either  side  of  the  vertical  centre  line,  and  4  feet  dis- 
tant from  it,  on  its  horizontal  axis. 

The  position  in  outline  of  the  central  casting  on  which  the  central  arm  of  one 
of  them  revolves  is  shown 
in  Fig.  116,   at   T,  by  a 
thick  dotted  line. 

Each  erector  con- 
sisted (see  Figs.  125  and 
1 26)  of  two  vertical  single 
acting  hydraulic  cylin- 
ders A,  A,  8 1  inches  in 
diameter,  having  a 


Jlr-on, 


— i 


FIG.  124.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Extension  Block  for  Additional  Rams. 

mon  piston  B,  on  which 

was  fixed  a  toothed  rack  (7,  operating  a  pinion  D,  which  when  revolved  carried 
with  it  a  cast  steel  box  E,  through  which  an  H  beam  F,  having  at  one  end  a  hand  H, 
to  which  could  be  secured  the  casting  to  be  lifted,  was  made  to  slide  by  means  of 
a  double-acting  hydraulic  cylinder  G,  also  secured  to  the  box  E.  By  admitting 
water  from  the  pressure  main,  which  for  these  erectors  was  at  about  1,000  pounds 
per  square  inch  to  the  cylinders  A ,  the  box  E  carrying  with  it  the  arm  F  was  made 
to  revolve,  and  by  pressure  supplied  through  a  swivel  joint  (see  Figs.  127  and  128) 
to  the  cylinder  G,  the  head  of  the  arm  F  could  be  advanced  or  withdrawn  from  the 
tunnel  lining,  the  full  extent  of  this  movement  being  6  feet  6  inches.  The  motion 
of  the  arm  F  was  therefore  in  a  vertical  plane  at  right  angles  to  the  axis  of  the 
tunnel,  but  this  arm  F  could  be  inclined  some  inches  from  this  vertical  plane  by 
an  adjusting  screw  and  hand  wheel,  by  means  of  which  the  position  of  the 
hinged  box  E  could  be  moved  relatively  to  the  shaft  on  which  it  and  the  pinion 
D  moved. 

The  arrangement  of  the  hydraulic  supply  pipes,  by  means  of  which  pressure 
was  conveyed  to  the  cylinder  G  of  the  revolving  arm  F,  was  ingenious. 

The  shaft  on  which  the  arm  and  the  pinion  D  turned  was  hollow  (see  the  en- 


FIG.   125.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Hydraulic  Erector. 

larged  section  of  this  part  in  Fig.  127),  and  through  it  the  supply  pipes  were  carried, 
the  detail  of  the  joints  at  X  and  Y  being  shown  in  Fig.  128. 

The  copper  pipes  connecting  the  joint  at  X  with  the  hydraulic  supply  main 
were  brought  to  the  point  X  on  the  front  side  of  the  back  diaphragm  of  the  shield, 

197 


TUNNEL    SHIELDS 


to  the  back  of  which  the  erector  was  fixed,  and  brought  through  it  at  X.  The  joint 
at  X  is  a  double  swivel  one,  similar  in  character  to  the  single  one  shown  in  Fig.  121, 

and  is  clearly  shown  in  Fig.  128.  The  brass 
collar  a,  to  which  the  supply  pipes  are 
secured,  is  of  course,  immovable,  and  in  it 
the  spindle  b  turns,  there  being  between  a 
and  b  the  circular  glands  c,  c  enclosing 
wrought-iron  rings  perforated  with  numerous 
holes,  the  other  end  of  b  terminating  at  Y  in 
a  rectangular  flange  d,  fitting  into  a  hole  cast 
for  the  purpose  in  the  hinged  box  E. 

In  b  were  cast  two  wrought-iron  pipes, 
j  inch  in  diameter,  which  made  a  connexion 
at  one  end  of  b  with  the  circular  joint  a, 
and  at  the  other  were  secured  in  a  brass 
boss  e  fastened  with  studs  to  b,  and  from 
which  copper  pipes  led  to  the  valves  of  the 
cylinder  G  of  the  arm  F  of  the  erector. 

When  therefore  the  arm  F  revolved  the 
spindle  6  turned  with  the  hinged  box  E,  a 
remaining  fixed,  and  the  pressure  water 
always  being  free  to  pass  through  one  or 
another  of  the  perforations  of  the  valve  ring 
at  c  c. 

These  erectors,  each  served  by  one  man, 
could  erect  one  complete  ring  weighing  nearly 
15  tons  in  about  an  hour,  and  occasionally, 
in  very  favourable  conditions  in  two-thirds 
of  that  time — a  very  satisfactory  rate  of  speed. 

Mr.  Moir's  own  criticism  of  his  machines 
is  as  follows  : — 


*•- H 

i  _'•;_-_-  .  . J-pJ-oi  _      _  _ _-  -_•„  J 


"S 
^    >> 

5  B 


82 


The  erectors  attached  to  the  back  of  the  shield 
had  worked  well,  but  on  another  occasion  he  would 
adopt  two  hemp-packed  cylinders  for  pushing  out 
and  pulling  in  the  telescopic  girder,  instead  of  one 
with  double  leathers,  which  were  more  trouble  to 
keep  in  order  than  common  glands.  Double  leathers 
inside  a  hydraulic  jack  always  gave  trouble  ;  they 
were  out  of  sight,  and  if  they  failed  the  whole 
machine  had  to  be  pulled  to  pieces  to  reach  them. 
Ordinary  hemp-packing  with  common  glands  was 
much  more  serviceable  in  ordinary  work.  In  de- 
signing the  erectors  he  had  allowed  a  margin  of 
50  per  cent,  in  the  power  of  the  telescope  cylinder, 
and  60  per  cent,  in  the  turning  cylinders.  He  might 
with  advantage  have  increased  that  allowance,  as 
sometimes  the  power  was  reduced  owing  to  leaky 
slide-valves,  the  surfaces  of  which  would  always 
wear. l 

The  details  of  these  erectors  were  in  a  large  measure  repeated  in  the  erector  of 
the  Great  Northern  and  City  Railway  Shield,  afterwards  employed  in  the  Holborn 
Subway  shield.2 

1  Proc.  Inst.   C.E.,  vol.  cxxx.  p.   80.  2  gee  p.    J28. 

198 


THE    SHIELD    IN    WATER-BEARING    STRATA 


The  lowering  of  the  shield,  which  to  advance  the  work  as  quickly  as  possible 
had  been  erected  on  the  surface  near  the  top  of  No.  4  shaft,  where  it  was  to  be  first 


FIG.   127.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    details  of  Hydraulic  Erector. 

employed,  was  effected  in  a  very  ingenious  manner.     The  great  weight  of  the 
machine,  some  200  tons  without  the  hydraulic  rams  and  other  fittings,  made  the 


XAxUf  Jiichea. 
?...;'          ,'         .'          *         ,' 


FIG.   128.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Enlarged  Details  of  the  Hydraulic  Pipes  of  Erector  at  X  and  Y  (Fig.  127). 

provision  of  tackle  to  lower  it  to  the  bottom  of  the  shaft  50  feet  below  a  very  serious 
matter,  and  it  was  finally  decided  (on  the  suggestion  of  Sir  W.  Arrol)  to  float  it 

199 


TUNNEL    SHIELDS 

down,  by  filling  the  shaft  with  water,  launching  the  shield  on  its  surface,  and,  by 
pumping  the  shaft  dry,  lowering  the  shield  gradually  to  the  bottom  (see  Fig.  129). 


M 


h?  O 

4 

W  "* 

o  13 


The  shield  was  erected  in  a  kind  of  dry  dock  adjoining  the  shaft,  the  upper 
part  of  which  was  constructed  so  as  to  permit  of  the  temporary  removal  of  suffi- 
cient iron  plates  to  allow  of  the  passage  of  the  shield. 


200 


THE    SHIELD    IN    WATER-BEARING    STRATA 

When  the  shaft  was  sunk,  and  the  shield  put  together,  the  shaft,  and  the  "  dry 
dock  "  adjoining  were  filled  with  water,  the  shield  having  been  made  into  a  water- 
tight cylinder  by  closing  its  ends  with  4-inch  planking  soundly  caulked. 

When  properly  ballasted,  the  shield  thus  made  buoyant  drew  about  17  feet  of 
water  and  could  be  drawn  through  the  side  of  the  caisson  into  it.  The  caisson  was 
then  pumped  dry  and  the  shield  lowered  on  to  a  cradle  previously  prepared  for  it. 

Behind  the  shield,  when  tunnelling  operations  were  in  progress,  a  travelling 
platform  or  gantry,  40  feet  long,  and  nearly  the  full  width  of  the  tunnel,  was  used, 
rails  for  its  support  being  fixed  on  brackets  in  the  tunnel  castings. 

It  was  attached  by  chains  to  the  shield  and  so  moved  forward  with  it.  It 
had  three  floors  from  which  the  whole  periphery  of  the  tunnel  save  the  invert  could 


FIG.   130.     BLACKWALL  TUNNEL,  LONDON. 
Travelling  Gantry  behind  Shield. 

be  reached  for  the  purposes  of  grouting  and  caulking,  and  on  which  the  spoil  from 
the  upper  platforms  of  the  shield  could  be  received.  This  platform  is  shown  in 
Fig.  130.1 

The  arrangements  for  working  with  compressed  air,  and  the  machinery  for 
supplying  it  were  very  complete,  and  in  particular  the  provisions  made  for  ensur- 
ing that  the  miners  employed  worked  under  the  most  satisfactory  conditions  as 
regards  their  health  were  thoroughly  efficacious,  with  the  result  that  the  work  was 
carried  through  without  loss  of  life,  and  indeed  almost  without  permanent  dis- 
ablement to  any  workman  due  to  the  natural  conditions  of  working  in  compressed 
air. 

The  machinery  for  the  supply  of  air  consisted  of  six  air  compressors,  of  a  total 
capacity  of  about  1,500  HP.  Of  these  the  two  largest  were  of  about  300  HP.  each. 

1  Figs.  130,  135,  138,  and  140  are  reproduced  from  the  Engineer  by  courteous  permission 
of  the  Editor. 

2O I 


TUNNEL    SHIELDS 

A  small  compressor  was  also  in  use  for  the  air  required  for  grouting  purposes. 
The  air  for  grouting  purposes,  the  pressure  of  which  was  about  40  pounds  per 
square  inch  greater  than  that  for  the  tunnel,  was  carried  in  a  5-inch  pipe.  The 
high-pressure  water  for  the  rams  was  carried  in  a  steel  pipe  1J  inch  in  diameter, 
and  as  the  end  of  the  pipe  was  connected  with  the  moving  shield,  a  "  walking  joint  " 
was  used  at  some  distance  back  from  the  face. 

Electric  incandescent  lamps  were  used  for  lighting  throughout  the  construction 
of  the  work  to  avoid  vitiation  of  the  atmosphere  by  lamps  or  candles  as  much  as 
possible,  and  electric  motors  driven  from  the  lighting  dynamos  were  used  to  operate 
drills  and  other  small  machinery  in  the  tunnels. 


FIG.   131.     BLACKWALL,  TUNNEL,  LONDON. 
Vertical  Airlock  for  Material. 


The  various  arrangements  for  giving  access  to  the  pressure  chamber  follow  in 
general  previous  equipments  for  similar  work,  but  present  some  special  features 
in  details. 

The  locks  for  giving  access  to  the  caissons  were  fixed  above  the  temporary 
airtight  floor  (see  Fig.  112),  and  were  two  in  number,  one  being  used  for  men  and  the 
other  for  material. 

The  former  does  not  call  for  any  special  remark  ;  the  latter  is  shown  in  Figs. 
131,  132,  133  ;  1  both  were  employed  on  one  of  the  large  caissons  at  the  Forth 
Bridge,  and  the  material  lock  was  again  erected  in  the  caissons  of  the  Greenwich 
Tunnel  in  1900. 

1  These  drawings  are  reproduced  from  Engineering,  by  courteous  permission  of  the  Editor. 

202 


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204 


THE    SHIELD    IN    WATER-BEARING    STRATA 

This  lock,  like  the  men's  lock,  was  fixed  in  the  top  of  a  shaft  3  feet  6  inches 
in  diameter,  and  consisted  of  a  box  at  top  of  the  3  feet  6  inch  shaft,  closed  at  top 
and  bottom  by  sliding  doors  A,  A.  These  doors,  when  opened,  slid  into  airtight 
recesses,  and  the  chamber  itself  was  large  enough  to  contain  on  one  side  of  the 
shaft  proper  the  drum  B  of  the  engine  for  raising  and  lowering  the  buckets.  The 
sliding  doors  were  worked  by  hydraulic  rams  C,  C,  or  alternatively  by  rack  and 
pinion  and  handwheel  E,  E.  By  an  interlocking  arrangement  of  simple  construction, 
D,  D,  Fig.  132,  it  was  made  impossible  that  both  doors  should  be  opened  at  once,  a 
very  necessary  precaution.  The  valves  admitting  water  to  the  rams  actuating  the 
doors  were  worked  by  the  handwheels  D,  D,  Figs.  131,  132,  133,  from  each  of  which  a 
segment  is  cut  out,  and  into  this  the  rim  of  the  other  wheel  fitted.  In  the  position 
shown  in  Fig.  132,  the  upper  valve  is  closed  and  its  handwheel  cannot  be  turned  un- 
less the  handwheel  of  the  lower  is  turned  round  to  bring  its  cutout  segment  opposite 
the  other  one.  In  that  position,  however,  the  lower  valve  would  be  closed  and  the 
lower  door  prevented  from  opening.  A  similar  adjustment  being  made  for  the 
valve  of  the  upper  door,  it  will  be  seen  that,  short  of  removing  the  special  hand- 
wheels  D,  D  altogether,  it  was  impossible  to  open  both  sliding  doors  at  one  time. 

The  winding  drum  B  was  driven  by  a  worm  F,  and  a  worm-wheel  G  outside 
the  lock,  and  actuated  by  a  pair  of  ordinary  reversible  engines  H,  H,  the  main 
shaft  which  carried  the  drum  being  provided  with  airtight  glands  at  both  ends 
where  it  passed  through  the  sides  of  the  airlock.  A  chain  passed  over  the  drum 
and  over  a  snatchblock  suspended  to  the  underside  of  the  upper  door  and  sliding 
in  and  out  with  it,  thus  bringing  the  point  of  suspension  to  the  centre  of  the 
shaft  for  lowering  the  skip  or  bucket  when  the  upper  door  was  shut,  and  when, 
consequently,  the  lower  door  could  be  opened. 

The  mode  of  working  these  locks  was  as  follows  :  A  crane  was  fixed  so  that  the 
end  of  its  jib  could  swing  exactly  over  the  centre  of  the  shaft,  and  when  it  was 
required  to  lower  an  empty  bucket  into  the  air  chamber,  the  bucket  was  hung 
over  the  upper  door  of  the  lock  by  this  crane  ;  the  upper  door  of  the  lock  was  then 
opened,  and  the  bucket  lowered  into  the  lock,  and  rested  on  the  lower  door,  and 
the  shackle  of  the  inside  winding  chain  attached  to  it.  This  done,  the  upper  door 
was  closed,  and  by  means  of  the  engines  the  bucket  lifted  clear  of  the  lower  door. 
A  cock  or  valve  J,  2%  inches  in  diameter,  which  communicated  with  the  compressed 
air  in  the  working  chamber,  was  opened  and  the  air  in  the  lock  put  at  the  same 
pressure  as  in  the  shaft  beneath. 

The  sliding  doors  were  fitted  with  india-rubber  joints,  and  the  effect  of 
admitting  the  air  from  below  into  the  lock  was  to  absolutely  seal  the  upper  door, 
and  when  that  was  secured,  a  turn  of  the  interlocking  wheels  D,  D  enabled  the 
lower  door  to  be  drawn  back,  and  the  bucket  was  lowered  to  the  bottom  of  the 
shaft  to  be  filled.  To  remove  the  loaded  bucket  the  process  just  described  was 
reversed  ;  the  bucket  was  drawn  up  into  the  lock,  the  lower  door  closed,  the  com- 
pressed air  let  out  of  the  lock,  the  upper  door  drawn  back,  the  winding  chain  taken 
off  the  bucket,  and  the  crane  chain  attached,when  the  crane  swung  away  the  bucket, 
discharged  its  contents  and  returned  the  empty  bucket  to  repeat  the  process. 

In  Fig.  132,  it  will  be  seen  that  the  bucket  when  rising  into  the  lock  touched 
a  lever  K,  which  opened  a  steam  whistle  and  so  indicated  to  the  men  outside  who  were 
working  the  lock  machinery  that  it  had  arrived  inside  the  lock.  As  an  additional 
precaution  a  small  dial  and  pointer  M  was  attached  to  the  worm-wheel  G,  so  that 
the  exact  position  of  the  bucket  was  indicated  at  any  moment  of  its  ascent  or  descent. 

205 


TUNNEL    SHIELDS 


The  levers  L,  L  worked  bolts,  which  fitted  in  the  frames  of  the  sliding  doors  and 
so  made  more  certain  that  both  doors  could  not  be  opened  together.  Locks  of  this 
character  cannot,  however,  be  worked  very  rapidly,  and  consequently  the  rate  of 
excavation  in  the  air  chamber  was  slow.  The  number  of  skips  sent  through  this 
lock  at  the  Forth  Bridge  was  about  twelve  per  hour,  and  later  at  Greenwich 
tunnel,  where,  however,  the  doors  were  worked  by  the  hand  gear  only,  not  more 
than  eight  per  hour  were  dealt  with. 

The  bulkheads  (see  Fig.  134)1  in  the  tunnel,  of  which  four  were  at  different 
times  constructed,  were  12  feet  6  inches  thick,  the  first  being  constructed  of  con- 
crete, and  the  others  of  brickwork,  the  latter  materials  presenting  less  difficulty 
when  the  bulkhead  came  to  be  removed.  All  were  rendered  with  cement  on  the 
pressure  side. 

The  working  locks,  two  in  number  in  each  bulkhead,  were  16  feet  long,  and 
7  feet  in  diameter,  the  skin  being  of  §  inch  plate.  The  use  of  this  thin  metal  was 
made  possible  by  the  fact  that  the  locks  were  made  to  pro- 
ject beyond  the  bulkhead  on  the  atmosphere  side  only,  and 
therefore  the  skin  of  the  lock  was  never  subjected  to  com- 
pressive  strain.  The  valves  for  working  them  were  respec- 
tively 2 1  and  1£  inches  in  diameter  for  locking  through 
materials  and  workmen  respectively. 

A  special  feature  in  these  locks  was  the  doors,  which 
were  hollow,  each  door,  which  had  an  area  of  20  square  feet, 
being  made  of  two  buckled  steel  plates,  ^  inch  thick, 
ri vetted  together,  thus  combining  lightness  with  strength. 

A  pipe  lock  18  feet  long  for  timber  and  rails  was  pro- 
vided. 

In  the  upper  part  of  the  bulkhead,  a  small  lock  for  use 
by  the  men,  in  case  of  flooding  of  the  tunnel,  and  accessible 
by  means  of  ladders,  was  provided. 
An  airtight  hanging  screen  or  diaphragm  (see  Fig.  135)  was  always  fixed  a 
short  distance  behind  the  shield  to  ensure  the  safety  of  the  men.  It  reached 
down  to  the  centre  of  the  tunnel,  and  was  fitted  with  an  airlock  at  the  top,  the 
doors  of  which  were  hung  to  open  towards  the  bulkhead  ;  this  could  be  reached 
by  a  gangway  which  was  added  to  as  the  shield  advanced.  In  the  event  of  an 
inrush  of  water,  the  men  could  thus  escape  to  the  other  side  of  the  screen,  where 
they  would  always  find  an  air-space,  and  thence,  by  a  gangway,  to  the  emergency 
lock  in  the  bulkhead. 

The  service  pipes  fixed  in  the  bulkheads  for  the  tunnel  work  were  two  8-inch 
air  pipes  for  the  supply  of  compressed  air  to  the  working  chamber,  one  5-inch  air- 
pipe  for  the  supply  of  compressed  air  at  60-70  pounds  pressure  for  grouting 
purposes,  three  5-inch  blow-out  pipes,  three  hydraulic  mains,  and  a  pipe  for  water 
at  ordinary  pressure  of  public  mains,  besides  electric  mains,  telephone,  etc. 

The  blow-out  pipes,  which  were  all  fitted  with  flexible  hoses  at  the  ends  for 
draining  away  the  water  in  the  invert  of  the  shield,  were  prolonged  as  the  shield 
advanced,  but  the  advantage  of  carrying  forward  the  supply  mains  is  not  obvious. 
By  bringing  them  forward,  the  air  in  the  rear  portion  of  the  pressure  chamber  was 
not  renewed,  save  to  the  extent  of  the  compressed  air  wasted  in  locking  through,  and 


FIG.  134.     BLACK  WALL 

TUNNEL,  LONDON. 
Bulkhead  in  Tunnel. 


1  The  various  pipes,  etc.,  are  not  shown  in  this  figure. 
206 


THE    SHIELD    IN    WATER-BEARING    STRATA 

the  increased  length  of  pipe  meant  increased  cost,  and  increased  loss  of  pressure 
by  friction. 

The  supply  pipes  appear  also  somewhat  small  in  the  light  of  the  experience 
gained  in  the  work. 

During  a  large  portion  of  the  time,  while  passing  through  loose  ballast,  as 
much  as  10,000  cubic  feet  of  air  per  minute  was  sent  into  the  tunnel.  It  was 
carried  into  the  tunnel  by  steel-rivetted  pipes  8  inches  in  diameter,  taken  down 
No.  4  shaft,  on  the  Kent  side,  and  thence  along  the  tunnel  as  completed  to  the  end 
at  the  Middlesex  side,  or  say  a  distance  of  nearly  3,000  feet  at  one  time.  When 
the  air  was  escaping  freely  at  the  face,  the  amount  pumped  through  these  pipes 
was  very  large,  and  the  velocity  consequently  very  high.  On  this  account  the 


FIG.   135.     BLACKWALL  TUNNEL,  LONDON. 
Safety  Diaphragm    in  Tunnel. 

difference  between  the  air-pressure  in  the  engine-house  and  the  tunnel  some- 
times showed  a  loss  of  40  per  cent.  It  is  evident  that  for  a  long  tunnel  it  would  be 
economical  to  have  pipes  of  ample  size,  so  that  the  velocity  of  air  could  be  kept  at 
say  about  30  feet  per  second,  as  the  extra  cost  of  pipes  would  soon  repay  itself,  and 
though  the  point  is  of  less  importance,  the  temperature  of  the  air  would  be  lower. 
A  good  plan  in  laying  out  the  pipe  arrangements  at  the  commencement  of  a 
tunnel  where  more  than  one  pipe  was  required,  would  be  to  make  the  length  of 
pipes  in  the  bulkhead  of  double  the  sectional  area  of  the  supply  mains  considered 
by  the  contractor  as  sufficient.  These  could  be  fitted  with  reducing  pieces  to 
suit  the  mains  in  use,  and  would  permit  of  the  whole  service  being  increased  later 
if  desired. 

207 


TUNNEL    SHIELDS 

All  the  haulage  work  in  the  tunnels  was  by  endless  ropes  driven  from  winches 
worked  with  compressed  air. 

After  the  shield  had  been  sunk  to  the  bottom  of  shaft  No.  4,  as  already 
described,  and  placed  in  position  for  entering  the  tunnel  opening  (towards  shaft 
No.  3)  in  which  cast-iron  guides  had  been  bolted  to  ensure  true  line  and  level  being 
followed,  a  portion  of  the  cast-iron  lining,  extending  to  the  other  side  of  the  shaft, 
was  temporarily  built  up  behind  the  shield  to  form  an  abutment  for  the  hydraulic 
rams  in  driving  the  shield  forward.  To  remove  the  plug  from  the  tunnel  opening 
a  commencement  was  made  at  the  bottom,  and  as  the  girders  carrying  the  bottom 
outside  plates  were  removed  the  latter  were  temporarily  strutted  to  the  shield. 
Clay,  chiefly  in  bags,  was  then  built  against  the  plates  to  support  the  face  when  they 
should  be  taken  out.  The  second  row  of  plates  being  similarly  dealt  with,  a  suffi- 
cient height  was  obtained  to  draw  out  the  bottom  row  by  means  of  a  tackle  or  union 
screw  ;  the  same  process  was  continued  with  the  other  plates  until  the  whole  of 
the  plug  was  removed  and  replaced  by  a  wall  of  clay,  through  which  the  shield  was 
driven  into  the  face  beyond.1  This  method  of  removing  the  plug  refers  more 
particularly  to  that  adopted  in  gravel,  etc.  ;  when  the  face  consisted  of  clay  such 
extreme  care  was  not  necessary.  The  ground  in  front  of  the  plug  was  sometimes 


SECTION     THROUGH  A.  A. 


FIG.   136.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Damage  to  Invert  at  Cutting  Edge. 

grouted  with  cement  before  the  plug  was  removed.  The  strata  on  starting  from 
No.  4  shaft  consisted  of  1  foot  of  sand  at  the  bottom  overlaid  by  25  feet  of  London 
Clay  with  about  1  foot  of  ballast  showing  at  the  top.  The  latter  had  been  drained 
to  a  large  extent  by  the  pumps  for  the  adjacent  "  cut-and-cover  "  work,  and,  as 
it  was  known  that  on  account  of  the  gradient  of  the  tunnel  the  ballast  would  soon 
disappear,  it  was  decided  not  to  use  compressed  air  at  the  outset,  but  to  drive  a 
top  heading  to  deal  with  the  gravel  and  water.  As  soon  as  the  clay  was  sufficiently 
thick  to  cut  the  water  off,  the  top  heading  was  discontinued. 

At  first  progress  was  somewhat  slow,  only  125  feet  being  driven  in  the  first 
two  months,  but  after  the  gravel  disappeared  and  the  top  heading  was  discon- 
tinued, better  progress  was  made,  an  average  length  of  25  feet  being  completed  per 
week.  An  accident,  however,  soon  after  happened  to  the  shield  which  caused 
some  delay.  At  the  base  of  the  London  Clay,  and  in  the  sand  immediately  below 
it,  large  pieces  of  rock  were  embedded  and  considerable  damage  was  caused  to 
the  cutting  edge  by  driving  against  them  (see  Fig.  136).  This  was  first  discovered 
after  fifty-four  rings  had  been  erected,  and,  although  great  care  was  exercised  in 

1  Compare  the  similar  operation  at  Greenwich  Tunnel,  page  240. 

208 


THE    SHIELD    IN    WATER-BEARING    STRATA 

clearing  the  excavation  in  front  of  the  upturned  part  of  the  cutting  edge,  the 
damage  continued  to  increase,  and,  after  another  twenty-six  rings  had  been  erected, 
the  shield  was  found  to  be  unworkable.  As  it  was  not  practicable  to  repair  it  in  its 
then  position,  it  was  decided  to  construct  a  concrete  cradle  for  it  to  slide  upon.  A 
timbered  heading,  19  feet  wide,  was  therefore  driven  and  kept  about  50  feet  in 
advance  of  the  shield,  so  that  the  concrete  should  have  time  to  become  hard  before 
the  shield  came  upon  it.  During  the  driving  of  this  heading  trouble  was  again 
experienced  from  water  in  the  ballast  above  finding  its  way  through  cracks  in  the 
clay,  and  the  top  heading  was  accordingly  recommenced,  so  as  to  intercept  the 
water  and  carry  it  through  the  shield.  This  method  of  working  was  continued 
until  No.  3  shaft  was  reached,  when  the  repairs  to  the  shield  were  effected. 

Until  about  490  feet  had  been  driven  towards  No.  3  shaft  no  great  quantity 
of  water  was  met  with,  but  at  that  point  a  large  volume  suddenly  broke  into  the 
bottom  heading.  Considerable  difficulty  was  then  being  experienced  in  sinking 
No.  3  shaft,  and  the  water  which  broke  into  the  heading  undoubtedly  came  from 
the  ballast,  and  found  its  way  either  down  the  side  of  the  shaft  or  through  the  cracks 
in  the  clay  which  had  been  caused  by  the  numerous  blows.  As  the  shield  was 
then  only  67  feet  from  the  shaft  (the  bottom  of  which  was  to  be  15  feet  below  the 
invert  of  tunnel)  it  was  deemed  prudent  to  suspend  any  further  tunnelling  opera- 
tions until  the  shaft  was  sunk  to  its  full  depth.  Meanwhile  the  first  bulkhead  was 
built,  and  No.  3  shaft  having  been  completed  to  its  proper  depth,  tunnelling 
operations  were  resumed  as  soon  as  the  bulkhead  was  completed,  the  remaining 
length  of  tunnel  from  this  point  to  the  shaft  being  driven  under  compressed  air. 

A  fire  which  occurred  in  the  top  heading  on  this  portion  of  the  work  caused 
considerable  anxiety.  It  was  feared  that  the  escaping  compressed  air  might  carry 
the  flames  through  the  ground  saturated  with  very  inflammable  material  to  a 
distillery  above,  in  which  case  a  serious  conflagration  would  have  resulted.  Happily 
a  good  supply  of  water  was  at  hand,  and  the  fire  was  extinguished  before  any  such 
accident  happened. 

The  air-tight  floor  was  fixed  and  pressure  applied  in  shaft  No.  3  when  it 
was  required  to  drive  the  shield  through  the  tunnel  opening. 

On  the  arrival  of  the  shield  at  the  shaft  it  became  necessary  to  undertake  the 
repairs  required  on  account  of  the  buckling  of  the  cutting  edge.  It  was  decided 
to  cut  away  the  distorted  portions  of  the  skins  and  vertical  stiffener,  Fig.  136,  and 
substitute  heavy  steel  castings,  as  shown  in  Fig.  139.  At  the  same  time  the  pro- 
jecting plates  of  the  undamaged  part  of  the  cutting  edge  were  cut  off,  and,  although 
sharpness  was  thereby  lost,  the  edge  was  much  stronger  to  sustain  a  blow  against 
any  obstacle  in  front.  The  steel  castings  were  made  to  a  larger  radius  than  the 
outside  skin,  so  that  they  should  stand  "  proud  "  of  it  and  thus  decrease  the  skin 
friction  when  the  shield  was  being  pushed  forward.  A  steel  band,  f  inch  thick, 
was  also  carried  round  the  outside  of  the  undamaged  part  of  the  cutting  edge,  with 
the  same  object,  and  to  further  strengthen  it. 

The  portion  of  the  tunnel  under  the  River  Thames  is  that  between  shafts 
Nos.  2  and  3,  a  distance  of  1,222  feet  (see  Fig.  111). 

Clay  was  found  over  the  tunnel  for  a  distance  of  about  700  feet,  where  it  was 
lost,  and  a  deep  pocket  of  ballast  going  down  far  below  the  invert  of  the  tunnel 
met  with.  The  work  was  carried  out  expeditiously,  and  without  difficulty  in  main- 
taining a  sufficient  pressure  of  air  as  long  as  the  clay  cover  continued.  In  start- 
ing from  No.  3  shaft  the  upper  part  of  the  shield  was  in  clay  and  the  lower  part  in 

209  P 


TUNNEL    SHIELDS 


sand.  The  latter  would,  without  air  pressure,  practically  be  a  quicksand,  but  with 
compressed  air  it  formed  most  favourable  material.  It  was  more  or  less  stratified 
and  interleaved  with  thin  beds  of  shale,  so  that  the  face  would  stand  with 
practically  no  timbering,  only  an  occasional  face-board  being  required,  and  being 
very  fine  and  close  there  was  little  escape  of  air.  The  rate  of  progress  here  surpassed 
that  in  any  similar  tunnel  previously  constructed.  In  two  months,  more  than  500 
feet  of  tunnel  were  completed,  and  occasionally  five  rings,  or  a  length  of  12  feet  6 
inches,  were  constructed  in  twenty-four  hours.  During  a  day,  therefore,  300  cubic 
yards  of  material  was  excavated,  and  about  75  tons  of  cast  iron  erected.  When 
it  is  considered  that  these  materials,  in  addition  to  lime,  other  necessaries  and 
empty  wagons,  had  to  pass  through  the  airlocks,  the  feat  appears  a  very  notable 
one,  involving  the  nicest  care  on  the  part  of  the  contractors  in  arranging  the 
work. 

The  bricks,  sand,  etc.,  for  No.  2  bulkhead,  which  was  built  under  the  river 
at  a  distance  of  about  220  feet  from  the  shaft,  had  also  to  be  brought  in  during 

this  time.  As  the  tunnel  approached  the 
centre  of  the  river  the  lower  part  passed 
through  mixed  deposits,  such  as  thin  beds  of 
clay,  clay  and  shells,  chalk  and  green-sand. 
Fig.  137  shows  a  characteristic  section  of  the 
strata  below  the  ballast.  The  excavation  of 
the  chalk  required  more  time  than  that  of 
the  sand,  but  still  the  progress  was  well 
maintained,  and  in  eleven  weeks  after  start- 
ing from  the  shaft  half  the  distance  across 
the  river  was  completed.  When  700  feet  had 
been  driven,  ballast  appeared  in  the  top,  and 
it  was  decided  to  stop  to  fix  the  shutters  at 
the  face  of  the  shield,  so  that  as  much  or  as 
little  of  the  face  could  be  closed  as  required  to 
prevent  undue  loss  of  air  and  any  sudden 
rush  of  ballast. 

Soon  after  entering  the  ballast,  which 
was  of  a  very  open  character,  the  shield 
passed  within  about  5  feet  of  the  original 
river  bed,  which,  however,  had  previously 

been  raised  by  tipping  into  the  river,  by  consent  of  the  Conservators  of  the  River 
Thames,  a  temporary  layer  of  clay  (see  Fig.  137)  for  a  distance  along  the  line  of 
the  tunnel  of  450  feet.  It  offered  resistance  to  the  air  escaping  from  the  tunnel 
through  the  open  ballast,  and  its  weight  prevented  the  bed  of  the  river  from  being 
blown  up  by  the  pressure. 

Without  the  provision  of  this  blanket  of  clay  it  would  have  been  impossible 
to  construct  the  length  of  tunnel  across  the  pocket  of  ballast.1  It  will  be  re- 
membered, that  after  the  accident  at  the  Thames  Tunnel  in  May,  1827,  the 
hole  caused  by  the  giving  way  of  one  of  the  frames  of  Brunei's  shield  was  filled  with 
clay,  through  which  the  shield  was  afterwards  driven. 

The  shutters  in  the  front  of  the  shield  (see  page  193  and  Figs.  119,  120  and 

1  Mr.  Moir  states  :  "  If  the  Thames  Conservancy  had  found  it  impossible  to  allow  the  river- 
bed to  be  raised,  it  had  been  intended  to  dredge  a  channel  and  fill  it  again  with  puddle." 

2IO 


CfreerLsarucL 


FIG.  137.     BLACKWALL  TUNNEL,  LONDON. 
Section  of  Tunnel  under  River  Thames. 


THE    SHIELD    IN    WATER-BEARING    STRATA 

138)  were  invaluable  in  the  work  in  ballast.      When  driving  the  shield  in  this 
material  the  procedure  was  as  follows  : — 

Previous  to  shoving  the  shield  forward,  the  face  of  a  compartment  was  completely 
closed  by  its  three  shutters  which  had  been  screwed  forward  as  close  to  the  cutting 
edge  as  possible,  the  shutters  being  directly  over  each  other,  and  the  small  space 
between  them  being  filled  with  clay.  When  the  shield  was  to  be  shoved  forward 
the  nuts  on  the  screws  were  loosened  on  the  forward  side  of  the  bearings,  allowing 
the  shutters  to  move  back  as  the  shield  was  shoved  forward.  Any  sudden  move- 
ment was  guarded  against  by  running  the  nut  only  1  inch  or  so  in  front  of  the 
bracket  at  a  time.  If  the  ground  was  very  loose,  and,  consequently,  the  air  escaped 
quickly,  a  man  in  each  compartment  kept  the  spaces  between  the  shutters  filled 
with  clay,  the  rate  of  travel  being  sufficiently  slow.  The  movement  of  the  shutters 


FIG.   138.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Sliding  Shutters. 

was  nearly  double  that  of  the  shield,  as  their  area  was  considerably  less  than  that 
enclosed  by  the  cutting  edge.  After  the  shield  had  been  shoved  as  far  as  required, 
the  shutters  would  be  considerably  behind  their  former  position,  the  space  in  front 
of  them  being  filled  with  ballast.  The  method  and  rate  of  removal  of  this  material 
depended  on  the  consistency  of  the  ballast,  and  whether  the  air  pressure  in  the 
tunnel  was  sufficiently  high.  Under  favourable  circumstances  the  ballast  might  be 
shovelled  out  from  the  top  of  the  shutters,  or  a  shutter  might  be  drawn  somewhat 
further  back  and  the  material  shoved  out  between  that  shutter  and  the  next  lower 
one  ;  but  in  any  case  the  top  shutter  was  first  excavated  and  screwed  forward  and 
then  those  at  the  middle  and  bottom.  When  the  gravel  was  very  coarse,  and  other 
circumstances  unfavourable,  all  the  ballast  in  front  of  the  shutters  was  scraped 
out  by  small  iron  rakes  or  by  hand  through  holes  7  inches  by  4  inches  in  the  shutters,1 

1  One  of  these  holes  is  shown  in  figure  138. 
211 


TUNNEL    SHIELDS 

which  were  furnished  with  sliding  doors,  the  greatest  care  being  taken  to  open  as 
small  an  area  of  the  face  as  possible,  and  each  shutter  being  screwed  up  as  the 
stuff  was  excavated.  It  is  not  surprising  that  when  working  in  this  way,  which 
was  often  necessary  for  safety,  the  progress  was  sometimes  only  1  foot  per  day. 
The  amount  which  the  shield  was  shoved  forward  at  one  time  varied  greatly  ;  some- 
times only  a  few  inches  could  be  obtained,  but  in  clay  the  length  necessary  for  a 
complete  ring  was  occasionally  completed. 

While  driving  through  the  ballast  the  cover  above  the  tunnel  was  so  small 
and  of  such  a  character  that  sufficient  pressure  could  not  be  maintained  to  dry  the 
ground  at  the  invert  ;  and  as  the  river  was  then  in  direct  communication  with  the 
face  a  large  and  constant  stream  of  water  found  its  way  into  the  bottom  floor  of 
the  shield.  It  was  dealt  with  by  blow-out  pipes,  four  5-inch  pipes  being  generally 
necessary  for  the  purpose.  These  pipes  had  flexible  hose  terminations  at  the 


FIG.   139.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Timberwork  in  Invert. 


shield,  and  the  water  was  blown  out  into  the  finished  part  of  the  tunnel  behind  the 
bulkhead  and  thence  pumped  to  the  surface. 

The  sliding  shutters  were  not  used  in  the  two  bottom  pockets,  and  in  their 
place  a  combination  of  long  horizontal  iron  piles  and  short  vertical  timber  piling 
with  horizontal  poling  boards  above  was  adopted.  In  Fig.  1 39  (the  left  hand  section) 
the  shield  is  ready  for  pushing  forward,  excavation  of  the  face  having  been  carried 
forward  to  the  line  of  the  cutting  edge,  and  timbered  by  short  piles  B  and  10  inch 
by  4  inch  polings  C,  C,  the  whole  secured  into  position  by  soldiers  D,  strutted  back 
to  a  byatt  E  fixed  in  the  frame  of  the  shield  by  the  telescopic  stretcher  or  "  gun  " 
F,  and  the  roof  being  secured  by  iron  needles  or  piles  H  fixed  under  the  framing  of 
the  platform  A,  and  by  means  of  head  trees  J  (of  old  rails)  resting  on  the  guides 
made  for  shutters  and  tailpieces  K. 

As  the  shield  was  pushed  forward  the  stretcher  F  was  allowed  to  slide  back, 
or  shut  up,  and  at  the  end  of  the  "  push  "  of  the  shield,  the  timbering  was  in  the 
position  shown  in  the  section  on  the  right  hand  of  Fig.  139.  By  this  arrangement 

212 


THE    SHIELD    IN    WATER-BEARING    STRATA 

it  was  possible  to  clear  out  the  ballast  up  to  the  cutting  edge,  which  could  not  be 
done  with  the  shutters.  Great  difficulty  was  experienced  in  the  bottom  floor, 
and  the  men  working  here  were  generally  standing  in  about  18  inches  of  water 
throughout  the  day.  Perforated  tunnel-plates  were  occasionally  built  in  to  relieve 
the  water  at  the  face  by  allowing  it  to  enter  the  tunnel  further  back.  Jets  of  high- 
pressure  water  were  used  at  times  at  the  face  to  keep  the  material  moving  while  the 
shield  was  advancing,  to  diminish  the  pressure  required  to  shove  the  shield  forward. 
Small  shots  were  also  used  to  shake  the  ground  and  to  remove  any  hard  material  ; 
and  a  man  was  constantly  employed  testing  the  face  with  a  bar  in  front  of  the 
shield  to  discover  any  large  boulders  which  might  damage  the  cutting  edge. 

Serious  "  blow  outs  "  occurred  while  working  in  the  ballast,  and  on  two  occa- 
sions the  air  pressure  fell  so  suddenly,  and  the  water  from  the  river  poured  in  so 
fast  in  the  two  upper  floors,  that  the  tunnel  was  flooded  to  a  depth  of  7  feet  or 
8  feet  in  a  few  seconds.  The  first  of  these  blows  occurred  on  April  30,  1895,  and 
the  second  and  largest  about  a  week  later,  when  about  three-quarters  of  the  distance 
across  the  river  had  been  covered.  The  escape  gangway  had  not  at  that  time 
been  provided,  and,  owing  to  the  dense  fog,  caused  by  the  sudden  fall  of  air  pressure, 
completely  obscuring  the  electric  lights,  the  men  had  difficulty  in  escaping  back 
to  the  lock,  then  distant  about  700  feet.  The  clay  which  had  been  placed  on  the 
bed  of  the  river  was  then  of  the  greatest  possible  value  ;  it  followed  down  upon  the 
ballast,  which  was  washed  in  by  the  water  as  through  a  funnel,  and  choking  the 
passage,  the  air  pressure  soon  increased. 

During  the  progress  of  the  shield  through  the  ballast,  careful  daily  observations 
were  made  both  of  the  material  in  front  of  the  shield,  by  pricking  ahead  from  the 
shield  itself,  and  also  of  the  clay  in  the  river  bed,  by  soundings  from  above. 

How  difficult  the  work  of  tunnelling  under  the  river  in  the  ballast  was  may  be 
inferred  from  the  fact  that  at  one  time  the  rate  of  progress  did  not  amount  to  more 
than  5  feet  a  week,  but  this  was  an  extreme  case,  and  as  the  depth  of  cover  increased 
on  approaching  the  north  shore  of  the  Thames  the  weekly  advance  was  about  20 
feet  per  week. 

The  total  time  occupied  in  tunnelling  from  shaft  No.  3  to  shaft  No.  2  (see 
Fig.  Ill),  was  fifty-four  weeks. 

When  the  shield  approached  No.  2  shaft,  and  before  removing  the  plug  to 
allow  of  its  entry,  iron  piles  were  driven  from  the  inside  of  the  shaft  over  the  top 
of  the  shield,  and  no  difficulty  was  experienced  in  driving  the  shield  into  the  shaft, 
and  connecting  up  the  tunnel  lining  with  the  iron  skin  of  the  caisson. 

The  driving  of  the  tunnel  from  shaft  No.  2  to  the  cut-and-cover  position  of  the 
work  presented,  for  the  first  part  of  its  length,  as  far  as  shaft  No.  1,  the  same  diffi- 
culties as  had  been  met  and  satisfactorily  overcome  before,  and  were  dealt  with  in 
the  same  manner. 

Beyond  shaft  No.  1,  the  difficulty  encountered  was,  for  the  most  part,  the  thin- 
ness of  the  cover  over  the  tunnel,  which  made  the  maintenance  of  sufficient  air 
pressure  to  keep  the  invert  dry  very  difficult. 

The  grouting  behind  the  cast-iron  tunnel  lining  was  carried  out  usually  a  few 
rings  behind  the  shield,  as  no  satisfactory  arrangement  could  be  made  to  prevent 
the  leakage  of  the  grout  into  the  tunnel  between  the  tail  of  the  shield  and  the  iron 
tunnel  lining.  In  the  large  shield  employed,  the  use  of  grouting  timbers  (see  page 
96)  was  impossible,  and  an  attempt  to  close  the  opening  at  the  tail  of  the  shield 
by  means  of  a  pneumatic  packing  was,  after  careful  trial,  abandoned. 

21?, 


TUNNEL    SHIELDS 

Neat  lias  lime  was  used  for  all  grouting  work. 

The  caulking  of  the  tunnel  lining  was  satisfactorily  carried  out  (see  page  62)  ; 
the  mixture  employed  being  J  pound  of  sal  ammoniac  to  100  pounds  of  iron  filings, 
lead  wire  being  caulked  into  the  joints  to  keep  them  dry  while  the  rust  concrete  set. 

The  contract  for  carrying  out  the  whole  of  the  tunnel  works  was  let  to  Messrs. 
Pearson  and  Son  in  1891,  and  the  whole  of  the  engineering  works  were  complete 
early  in  1897,  the  cost  being  about  £850,000. 


FIG.   140.     BLACKWALL  TUNNEL,  LONDON. 
The  Shield  :    Back  View. 

The  cost  per  yard  of  the  iron-lined  tunnel  was  £378  where  the  heavy  section 
of  cast-iron  lining  was  used,  and  £315  where  the  shallower  castings  were  put  in, 
the  cost  of  the  roadway  and  internal  concrete  and  tile  lining  not  being  included. 

The  London  County  Council  showed  great  interest  in  the  health  of  the  men 
employed  in  compressed  air  ;  and  a  stringent  clause  was  inserted  in  the  contract 
specifying,  among  other  particulars,  proper  ventilation  at  the  working  face,  the 
provision  of  lifts  and  resting-places  for  the  men,  including  a  compressed  air  chamber 
fitted  with  locks,  and  that  no  workman  should  be  engaged  for  compressed  air-work 
without  his  fitness  for  such  duties  being  proved  by  previous  experience  or  by  medical 
examination.1  After  the  commencement  of  the  work  the  Council  obtained  parlia- 

1  For  observations  made  at  this  tunnel  on  compressed  air  conditions,  see  p.  41. 

214 


THE    SHIELD    IN    WATER-BEARING    STRATA 

mentary  powers  to  compensate  men  who  might  be  injured  by  working  in  com- 
pressed air,  although  the  men  were  not  employed  directly  by  the  Council.  A  resident 
medical  officer  was  also  appointed  to  examine  all  men  previous  to  working  in  the 
compressed  air,  or  who  might  be  injuriously  affected  by  it,  and  to  thoroughly  in- 
vestigate the  nature  of  compressed-air  illness.  The  cases  of  illness  due  to  com- 
pressed air  were  few,  and  only  three  cases  of  permanent  illness  have  been  attributed 
to  this  cause,  and  there  were  no  deaths.  This  is  probably  due  to  the  fact  that  a 
sufficient  supply  of  air  was  delivered,  and  that  the  men  did  not  work  in  too  long 
shifts.  The  general  period  of  work  was  eight  hours,  but  there  was  a  break  of  three- 
quarters  of  an  hour  in  this  time  as  near  the  middle  as  possible.  Hot  coffee  was 
supplied  by  the  contractors  to  the  men  in  the  middle  of  their  shift,  and  also  before 
coming  out  ;  and  proper  drying  rooms  with  lockers  were  provided  for  their  clothes. 


The  East  River  Gas  Tunnel,  New  York  (1892) 

In  1892,  the  East  River  Gas  Company,  the  works  of  which  are  situated  on  the 
Long  Island  side  of  the  East  River,  commenced  an  extension  of  their  supply  system 
to  the  city  of  New  York  on  the  other  side  of  the  river.  The  principal  engineering 
feature  in  this  extension  was  the  construction  of  a  tunnel  1  under  the  East  River 
for  the  conveyance  of  the  gas  mains  to  the  other  side,  and  from  the  results  obtained 
by  trial  borings  on  the  line  selected  for  the  tunnel  it  appeared  that  rock  would  be 
met  with  for  the  entire  length  across  the  river  ;  and  on  this  supposition  a  contract 
for  the  work  was  let. 


Long  /. 


FIG.   141.     EAST  RIVER  TUNNEL,  NEW  YORK. 
Longitudinal  Section. 

A  longitudinal  section  of  the  tunnel  is  shown  in  Fig.  141,  and  its  internal 
dimensions  were  originally  fixed  at  10  feet  6  inches  width  and  8  feet  6  inches  height 
to  accommodate  one  main  4  feet  in  diameter,  and  two  each  3  feet  in  diameter.  Its 
length  between  shafts  is  2,500  feet. 

The  tunnel  was  designed  to  be  on  a  gradient  of  1  in  200,  falling  towards  Long 
Island,  the  minimum  cover  over  the  roof  of  the  tunnel  being  41  feet  on  the  New 
York  side  of  the  river  and  85  feet  on  the  Long  Island  side.  In  the  centre,  for  a 
length  of  about  800  feet,  the  tunnel  is  under  Blackwell's  Island. 

Work  was  commenced  by  sinking  shafts  at  each  end  of  the  proposed  tunnel, 
rock  being  in  both  cases  met  with  about  8  feet  from  the  surface,  and  tunnelling 
was  commenced  in  hard  gneiss  rock,  that  in  the  Long  Island  side  being,  however, 
fissured,  and,  in  consequence,  making  the  work  of  excavation  expensive  and  slow 
by  reason  of  the  pumping  required. 

At  a  distance  of  338  feet  from  the  New  York  shaft,  a  fissure  in  the  hard  gneiss 
was  struck,  and  salt-water  came  in  some  quantity  into  the  tunnel.  On  pushing  on 

1  The  description  of  these  works  is  mainly  taken  from  Mr.  W.  L.  Aims'  Paper,  "  Notes  on 
the  Construction  of  the  East  River  Gas  Tunnel,"  read  before  the  Boston  Society  of  Civil  En- 
gineers, April  17,  1895. 

215 


TUNNEL    SHIELDS 

further,  the  hard  gneiss  became  softer  and  more  micaceous,  and  at  about  360  feet 
from  the  shaft  the  material  cut  through  changed  in  character,  a  vein  of  soft  material 
being  met  with,  with  the  same  dip  and  strike  as  had  the  rock  previously  passed 
through,  of  which  it  was  no  doubt  a  decomposed  vein.  In  composition  it  was  a  de- 
composed feldspar,  crumbling  easily  with  no  perceptible  grit,  and  perfectly  dry  when 
undisturbed  ;  but  it  was  easily  acted  on  by  water,  which  it  absorbed,  and  which 
caused  it  to  break  away  over  the  surfaces  exposed  to  the  air.  On  either  side  of  this 
soft  material  at  its  junction  with  the  hard  gneiss,  water  was  found  in  considerable 
quantity,  and  it  was  this  fact  which  made  the  use  of  compressed  air  a  necessity. 

An  endeavour  was  made  by  driving  a  heading,  6  feet  high  by  4  feet  wide,  in 
advance  to  ascertain  the  extent  of  the  soft  vein,  but  had  to  be  abandoned  owing 
to  the  increase  in  the  quantity  of  water  which  found  its  way  in  at  the  face.  The 
action  of  the  water  in  the  soft  material  was  curious  in  its  effects. 

The  water  running  along  the  face  of  the  (hard)  rock  had  washed  out  a  cavity  above  the 
tunnel  in  the  soft  ground.  The  walls  of  this  cavity  were  gradually  breaking  away,  and  the 
clay-like  substance  falling  down  would  close  the  outlet  of  the  water  into  the  tunnel.  The 
water  would  then  accumulate  in  this  pocket,  softening  up  fresh  material  on  the  sides  (of  the 
pocket),  until  it  had  gained  a  sufficient  head  to  burst  through  the  dam  which  confined  it,  when 
it  would  come  rushing  into  the  tunnel,  carrying  with  it  large  quantities  of  the  softened  material. 
These  rushes  were  accompanied  by  a  loud  bubbling  sound,  that  quite  mystified  the  men,  which 
was,  of  course,  the  sound  of  the  air  displacing  the  water  in  the  cavity.  As  soon  as  the  pocket 
had  emptied  itself,  for  a  time  the  trouble  was  over,  until  with  the  falling  of  more  material  the 
outlet  was  again  closed,  and  the  operation  was  repeated. 

Ultimately  a  bulkhead  was  hurriedly  constructed  across  the  face  of  the  tunnel, 
hay  being  used  in  front  of  it  to  prevent,  as  far  as  possible,  the  washing  out  of  the 
soft  material. 

It  was  under  consideration  to  abandon  the  length  of  tunnel  already  constructed 
and  to  sink  the  shaft  to  a  greater  depth,  but,  from  the  fact  that  the  dip  of  the  de- 
composed was  so  nearly  vertical,  there  was  little  reason  to  suppose  that  at  any 
reasonable  depth  better  conditions  would  obtain.  The  continuance  of  the  tunnel 
already  commenced  was  therefore  resolved  on,  but  with  the  aid  of  compressed  air. 
The  depth  of  the  tunnel  invert  at  the  centre  of  the  New  York  channel  of  the  river 
was  nearly  120  feet  below  mean  high  water  and  at  the  centre  of  the  Ravenswood 
channel  about  127  feet  ;  the  theoretic  air  pressure  required  to  hold  back  the 
superincumbent  water  was  therefore  55  pounds.  In  actual  working  a  pressure  of 
48  pounds  was  sometimes  employed,  or  80  per  cent,  of  the  calculated  amount  for 
the  depth,  a  proportion  that  shows,  with  such  an  amount  of  cover,  that  very  large 
and  open  fissures  must  have  existed  to  admit  water.  An  airlock  and  bulkhead 
were  fixed  in  the  tunnel  about  40  feet  behind  the  soft  rock  face  described  above, 
the  bulkhead,  8  feet  thick,  being  built  into  chases  cut  in  the  rock  ;  and  the  usual 
equipment  of  compressors,  electric  light,  etc.,  was  installed. 

The  operations  up  to  this  date,  including  the  sinking  of  the  shaft,  had  taken 
from  July  1892  to  February  1893.  On  the  25th  of  that  month,  work  was  resumed 
under  an  air  pressure  of  38  pounds,  which  was  soon  raised  to  42  to  cope  with  the 
water  which  came  in  through  the  ground  disturbed  by  the  driving  of  the  earlier 
advance  heading. 

The  excavation  was  advanced  under  a  cylindrical  steel  roof,  built  up  of  plates 
3  feet  long  and  1  foot  wide,  of  f  inch  sheet  steel,  to  the  four  sides  of  which  were 
rivetted  angle  bars  2£  inches  by  2|  inches  by  J  inch.  These  plates  were  bolted 
together  in  a  heading  about  6  feet  high.  In  the  erection  of  this  roof,  poling  boards 

216 


THE    SHIELD    IN    WATER-BEARING    STRATA 


were  used  for  each  plate,  and  a  bulkhead  carried  down  with  each  ring  as  erected. 
When  the  heading  had  been  advanced  about  20  feet  from  the  rock,  a  12  inch  by 
12  inch  yellow  pine  mudsill  or  long  foot  block  (see  Fig.  142)  was  introduced  along 
the  bottom  of  the  heading,  and  on  this  the  roof  was  carried  by  means  of  radia 
timber  bracing.  The  excavation  was  now  carried  down  on  both  sides  of  this  mudsill, 
to  a  distance  of  about  10  feet  from  the  rock,  the  steel  roof  being  extended  well  down 
on  the  sides.  A  circular  section  was  thus  excavated,  in  which  brickwork  was  laid, 
four  courses  thick,  and  with  an  internal  diameter  of  10  feet.  Between  March  4 
and  16  a  great  deal  of  trouble  was  experienced.  Air  pressure  was  several  times  to 
48  pounds,  and  the  work  progressed  very  slowly  on  account  of  the  many  inrushes  of 
water,  and  softened  material.  It  was  not  until  April  8  that  the  last  section  of 
brickwork  in  the  soft  material  was  completed,  and  rock  again  entered,  after  passing 
through  29  feet  of  this  decomposed  material.  Of  the  material  met  in  driving 
through  this  vein,  at  first  9  feet  of  the  grey  decomposed  feldspar  was  penetrated, 
a  vein  of  4  inches  of  hard  quartz  was 

then  met,  and  this  was  followed  by  6  feet  'Scu^ 

of  pure  white  decomposed  feldspar, 
smooth  and  soft  as  plaster.  The  re- 
maining 14  feet  was  made  up  of  layers 
of  feldspar  and  chlorite.  This  chlorite, 
deep  green  in  colour,  flaky,  and  grease- 
like  to  the  touch  when  wet,  proved  to  be 
very  troublesome  material,  as  it  was 
easily  converted  into  a  fluid  state  by  the 
water,  which  was  again  encountered  next 
to  the  rock. 

The  rock  encountered  beyond  the 
soft  seam  closely  resembled  the  decom- 
posed material  which  had  been  pene- 
trated before,  and  consisted  of  alternate 
layers  of  feldspar  and  chlorite  with  an 
occasional  vein  of  quartz.  It  was  quite 
soft,  though  requiring  drilling  and  blast- 
ing, and  eventually  it  had  to  be  lined. 

After  the  heading  had  been  driven  about  69  feet  into  this  rock  the  company  decided, 
in  spite  of  the  uncertainty  as  to  the  material  ahead,  to  remove  the  air  pressure.  On 
this  being  done,  however,  the  brickwork  through  the  soft  seam  proved  so  unsatisfac- 
tory in  excluding  the  water,  that  air-pressure  was  again  put  on,  and  it  was  decided  to 
line  the  brickwork  with  a  circular  cast-iron  lining.  Although  this  brickwork  was 
only  10  feet  in  inside  diameter,  a  lining  was  designed  10  feet  2  inches  in  the  clear,  as 
it  was  now  desired  to  make  the  tunnel  bore  as  large  as  possible.  To  put  in  this  lining, 
some  of  the  brickwork  had  to  be  cut  out,  which  was  then  removed  in  sections, 
enough  for  one  ring  of  plates  at  a  time.  The  lining  consisted  of  rings  of  plates  or 
segments,  each  segment  being  about  3  feet  long  and  1  foot  4  inches  wide,  with  inter- 
nal flanges  4  inches  deep,  from  the  back  of  the  plate.  The  metal  in  both  the  back 
of  the  plate  and  the  flanges  was  1£  inches  thick.  All  the  joint-faces  of  the  segments 
were  planed  and  1-inch  bolts  used  for  fastening  them  together.  A  complete  tunnel 
ring  was  composed  of  nine  segments  and  a  small  inverted  key,  about  8  inches  wide. 
The  weight  was  a  little  more  than  1  ton  per  lineal  foot  of  tunnel.  The  work  of 

217 


FIG.   142.    EAST  RIVER  TUNNEL,  NEW  YORK. 
Iron  Polings  of  Roof. 


TUNNEL    SHIELDS 

putting  the  cast-iron  lining  into  the  brickwork  was  necessarily  a  very  slow  opera- 
tion. The  lining  was  extended  well  into  the  rock  on  both  sides  of  the  soft  vein, 
and  a  wall  built  at  both  ends  between  the  rock  and  the  iron  lining,  to  confine  the 
Portland  cement  grout  which  was  now  introduced  back  of  the  plates.  To  effect 
this  grouting  l£-inch  holes  had  been  drilled  and  tapped  through  the  back  of  several 
plates  in  each  ring.  Through  these  holes  the  grout  was  pumped,  and  after  the 
space  between  the  brickwork  and  the  lining  had  been  thoroughly  grouted,  the 
work  was  found,  on  taking  off  the  air  pressure  from  the  heading,  to  be  perfectly 
water-tight.  It  was  not  until  toward  the  end  of  July  that  the  work  of  lining  the 
brickwork  was  completed  and  driving  ahead  in  the  rock  was  resumed.  Then, 
when  an  advance  of  only  10  feet  had  been  made,  a  second  soft  seam  was  encountered 
about  80  feet  beyond  the  first  one,  and  a  test  pipe  was  driven  to  a  distance  of  70 
feet  without  encountering  anything  solid,  the  material  consisting  of  decomposed 
feldspars  and  chlorites,  alternating  with  black  mud  and  sand. 

Water  was  again  found  next  to  the  rock,  but  was  to  some  extent  held  in  check 
by  the  compressed  air.  As  from  the  results  of  the  test  pipe  there  were  no  special 
difficulties  to  apprehend  from  the  indicated  material,  it  was  decided  to  drive  ahead, 
under  the  open  heading  method,  as  this  involved  no  delays  in  waiting  for  special 
machinery.  The  light  steel  cylindrical  roof  was  again  used  in  advancing  the  exca- 
vation, but  for  the  permanent  lining  the  cast-iron  rings  were  to  be  introduced  in- 
stead of  brickwork,  as  heretofore.  A  start  was  made  on  August  7  to  drive  the  head- 
ing into  the  soft  material,  but  two  days  later,  after  the  work  had  been  advanced 
6  feet  into  the  soft  vein,  orders  were  received  to  suspend  all  work  on  account  of  the 
great  financial  depression  of  the  time.  This  was  unfortunate,  and  could  it  have  been 
anticipated  a  few  days  the  heading  into  the  soft  material  would  have  been  left  un- 
opened. As  it  was  now,  from  being  first  disturbed  and  then  abandoned,  the  water 
was  allowed  to  soften  up  the  black  mud  in  the  heading,  and,  in  spite  of  the  bulkhead, 
a  considerable  quantity  of  the  material  was  washed  into  the  tunnel. 

By  this  time  some  progress  had  been  made  with  tunnel  work  from  the  shaft 
at  Ravenswood,  Long  Island.  Work  on  this  shaft  had  commenced  in  June,  1892, 
and  by  March,  1893,  about  290  feet  of  tunnel  was  built,  the  material  met  with 
being  a  hard  seamy  gneiss  interrupted  by  seams  of  soft  chlorite  about  4  feet  thick. 
At  this  point,  however,  a  vein  of  almost  liquid  chlorite  was  found  by  test  drills 
about  2  feet  ahead  of  the  face. 

These  holes  were  plugged,  but  as  it  was  necessary  to  know  what  was  ahead, 
and  as  with  100  feet  of  cover  between  the  tunnel  roof  and  the  river  bottom  it  was 
thought  that  the  condition  of  affairs  could  not  be  very  serious,  it  was  decided  to 
continue  driving  ahead  without  air  pressure,  and  with  a  timbered  heading.  To  see 
what  the  material  would  do,  several  hand-holes  were  put  into  the  rock-face  with  the 
object  of  blasting  out  a  hole  about  2  feet  square  through  the  remaining  2  feet  of 
rock,  to  the  chlorite.  Before  blasting,  however,  the  precaution  was  taken  to  build 
a  bulkhead,  some  40  feet  back  from  the  face.  On  firing  the  holes,  an  inrush  of 
many  yards  of  material  took  place,  which  was  finally  checked  by  some  rock  frag- 
ments closing  the  opening  through  the  rock. 

Repeated  attempts  to  work  a  timbered  heading  in  this  material  proved  un- 
successful, and  in  March,  1903,  work  was  abandoned,  and  the  shaft  and  tunnel 
were  drowned  out.  When  the  stoppage  of  work  in  the  New  York  heading,  due  to 
financial  difficulties,  occurred,  work  in  the  other  tunnel  had  been  at  a  standstill  for 
five  months,  except  that  an  airlock  and  bulkhead  had  been  put  in,  and  a 

218 


THE    SHIELD    IN    WATER-BEARING    STRATA 

trial  hole  driven  in  the  face  which  disclosed  the  fact  that  the  seams  of  chlorite 
and  other  soft  matter  was  32  feet  thick,  and  that  beyond  was  a  soft  white  limestone. 

When  the  company  were  in  a  position  to  resume  tunnelling  operations,  it 
was  resolved  to  employ  shields,  and  as  the  bulkheads  and  airlocks  were  already 
built  in  the  tunnel  it  was  necessary  to  design  the  details  of  the  machines  so  as  to 
admit  of  all  the  parts  passing  through  the  airlocks,  and  it  was  thought  convenient 
also  to  make  them,  as  far  as  was  compatible  with  the  necessary  stiffness,  easy  to 
strip  and  re-erect,  so  as  to  admit  of  the  shields  being  taken  to  pieces  and  re-erected 
with  ease. 

Figs.  143  and  144  1  show  the  shields  as  designed  by  Mr.  Aims  to  meet  these 
requirements.  To  avoid  having  the  plates  of  the  skin  of  the  usual  large  size,  the 
length  of  the  skin  plate  being  generally  the  length  of  the  shield,  he  divided  the 


•sionP/are 
'JacHSpace 


Longitudinal  Section .  End  View  of  Head . 

FIG.   143.     EAST  RIVER  TUNNEL,  NEW  YORK. 
The  Shield. 


shield  transversely,  separating  the  tail-end  section,  or  that  which  overlaps  the 
tunnel,  from  the  cutting  edge  section  containing  the  working  chambers.  These 
two  sections  were,  of  course,  circular,  11  feet  f  inch  outside  diameter.  The 
tail-end  section  was  3  feet  6  inches  long,  and  the  cutting-edge  section  3  feet  8  inches 
long.  Both  of  these  sections  were  again  divided,  longitudinally,  into  four  quad- 
rants. The  outside  shell,  in  both  tail-end  and  cutting-edge  sections,  was  made  up 
of  one  J-inch  and  one  f-inch  steel  plates  rivetted  together,  and,  at  the  four  quadrant 
joints,  there  were  ^-inch  butt-straps  12  inches  wide  running  the  whole  length  of 
the  shield  and  uniting  the  quadrants  and  the  two  sections.  The  middle  diaphragm, 
separating  the  cutting-edge  and  tail-end  sections,  was  made  of  two  plates,  one 

1  Reproduced  from  Engineering  News,  March  1,  1894,  by  courtesy  of  the  Editor. 

219 


TUNNEL    SHIELDS 


rivetted  to  each  of  the  two  sections,  and  these  two  plates  bolted  together  with  the 
butt-straps  united  the  sections. 

The  cutting-edge  section  contained  two  stiff eners,  one  vertical  and  one  hori- 
zontal, of  the  same  length  as  the  section. 

Within  the  skin  of  the  front  section  of  the  shield  was  built  a  circular  box  girder, 
the  inside  flange  of  which,  f  inch  thick,  had  a  diameter  of  9  feet.  In  this,  in  com- 
partments formed  by  plate  gussets  and  angles,  were  twelve  hydraulic  rams. 

The  cutting  edge  extended  1  foot  beyond  the  box  girder  containing  the  rams, 
and  was  stiffened  by  inclined  plates  from  the  flange  of  the  girder  to  the  skin. 

The  horizontal  stiff ener,  or  platform,  in 
the  cutting  edge  section  of  the  shield,  in  front 
of  the  main  diaphragm,  had  provided  a  detach- 
able extension  in  front  which  extended  2  feet  8 
inches  in  front  of  the  cutting  edge,  and  served 
the  same  purpose  as  the  sliding  tables  on  the 
larger  shields  used  in  London  Clay  tunnelling, 
namely  to  hold  up  the  face,  and  to  afford  a 
working  area  for  the  miners  when  taking  out 
the  excavation  at  the  top. 

To  erect  this  shield  the  only  rivetting 
necessary  was  at  the  four  but-strap  joints  in 
the  tail-end  section,  where  it  was  necessary  to 
preserve  a  flush  surface  on  both  sides  of  the 
outer  shell.  In  the  cutting  edge  part  counter- 
sunk bolts  were  used  through  the  butt-straps. 
About  380  |-inch  bolts  and  160  rivets  were 
used  to  erect  the  shield.  Two  doors  closing 
each  of  the  four  working  chambers  were  hung 
on  the  vertical  platform,  and  were  provided 
with  fastenings  so  that  the  whole  face  could 
be  easily  closed. 

This  arrangement  resembles  that  adopted  for  the  Greenwich  tunnel  shield  in 
the  later  stages  of  that  work  (see  Figs.  163  and  164). 

To  drive  the  shield  twelve  5-inch  hydraulic  jacks  were  used,  designed  for  a  work- 
ing pressure  of  5,000  pounds  per  square  inch,  or  600  tons  on  the  whole  shield  (Fig.  144). 
These  jacks  were  controlled  by  two  block- valves,  one  placed  on  each  side  of  the 
shield.  Each  of  these  block- valves  consisted  of  six  independent  valves  all  in  one 
compact  casting,  each  of  which  had  a  pressure  and  exhaust  stem.  Half -inch 
pipe  was  used  for  connecting  each  jack  with  its  valve,  and  1-inch  hydraulic  pipe 
was  used  for  the  pressure-main  which  was  connected  with  the  shield  block- valves 
by  three  swivel-joint  connexions.  To  furnish  the  pressure,  a  very  compact  little 
pump,  was  used  without  an  accumulator,  the  pressure  being  governed  by  a 
steam-regulating  valve. 

On  September  22  work  was  resumed  on  the  New  York  side,  with  a  small  force 
of  men  working  days  only,  to  excavate  in  the  rock  an  enlarged  chamber  about 
15  feet  back  from  the  face,  in  which  to  erect  the  shield.  This  chamber  was  made 
circular  about  15  feet  in  diameter  and  10  feet  long.  Back  from  this,  the  rock 
was  taken  out  in  a  circular  form  of  about  11  feet  diameter,  for  some  14  feet,  or 
enough  for  about  10  rings  of  the  cast-iron  segments  which  were  here  erected  in 

220 


FIG.  144. 


EAST  RIVER  TUNNEL,  NEW 
YORK. 


The  Shield  :    Hydraulic  Rams. 


THE    SHIELD    IN    WATER-BEARING    STRATA 

the  rock,  the  spaces  between  being  thoroughly  grouted  with  Portland  cement. 
These  rings  were  thus  made  solid  in  the  rock  to  withstand  the  thrust  of  the  shield- 
jacks  upon  the  lining.  The  blasting  necessary  in  this  work  was  made  as  light  as 
possible,  but  it  was  not  without  its  effect  upon  the  soft  material  in  the  heading, 
a  considerable  quantity  of  the  black  mud  being  washed  through  the  bulkhead, 
while  the  braces  showed  signs  of  a  heavy  strain  from  the  squeezing  of  the  material. 
The  shield  arrived  at  the  works  on  November  10,  and  the  work  of  erection  was 
immediately  begun.  The  sections  were  lowered  down  the  shaft  and  taken  through 
the  airlock  to  the  shield-chamber.  On  November  17  the  shield  was  all  assembled, 
and  ri vetting  the  tail-end  sections  was  commenced.  For  heating  the  rivets  in  the 
air-chamber  a  forge  was  used,  with  a  hood  to  which  was  connected  at  the  top  a 
2-inch  pipe  with  a  valve  which  extended  through  the  air-lock  bulkhead.  By  means 
of  this  pipe  all  the  obnoxious  gases  from  the  furnace  were  removed  from  the  air- 
chamber.  After  the  rivetting  was  finished,  the  shield  was  brought  to  its  right 
position  for  line  and  gradient,  and  all  the  machinery  of  the  rams  fitted  to  it  ;  the 
length  of  cast-iron  lining  already  erected  was  extended  to  reach  under  the  tail  of 
the  shield,  and  the  advance  was  commenced.  Continuous  work,  in  eight-hour 
shifts,  was  arranged  for. 

The  shield  was  advanced  until  it  was  necessary  to  disturb  the  bulkhead,  the 
remaining  bench  ahead  of  the  shield  being  blasted  out  as  the  shield  progressed. 

The  most  difficult  part  of  the  work  was  then  reached,  for  at  the  point  where 
the  shield  entered  the  soft  black  mud  on  top  there  still  remained  about  12  feet  of 
hard  rock  in  the  bottom,  as  the  dip  of  this  vein  was  over  40  degrees  towards  Long 
Island.  Blasting  had  therefore  to  be  continued  in  the  bottom  pockets  of  the  shield 
after  the  top  had  entered  the  much  softened  material.  As  soon  as  the  bulkhead 
was  passed  it  was  with  great  difficulty  that  the  bottom  pockets  could  be  kept  clear 
of  the  black  slush  from  overhead.  The  material  had  become  so  softened  along  the 
rock  face  that  it  was  almost  impossible  to  confine  it,  and  several  rushes  of  inflowing 
material  occurred,  until  finally  an  open  connexion  with  the  river  was  established, 
and  the  tunnel  was  visited  by  crabs  and  mussels,  together  with  boulders,  old  boots 
and  shoes,  brick,  and  tinware  direct  from  the  river  bottom.  Notwithstanding 
these  adverse  circumstances  the  work  went  on,  although  in  45  pounds  of  com- 
pressed air,  which  was  now  escaping  through  the  heading  and  causing  a  very 
violent  ebullition  on  the  river  surface.  This  upward  current  of  air  held  in  check 
the  downward  current  of  water,  so  that  no  efforts  were  made  to  prevent  its  escape. 
On  December  13  the  shield  finally  cleared  the  rock  and  was  now  fully  entered 
into  the  soft  black  mud.  The  main  difficulty  now  surmounted,  the  work  progressed 
more  rapidly,  and  the  shield  soon  reached  undisturbed  material,  a  black  mud, 
dry  and  hard,  with  occasional  lumps  like  charcoal  and  numerous  nodules  of  pyrites. 
Mattocks  were  used  by  the  men  in  the  working  chambers,  who  would  clean  out 
these  four  compartments  to  within  a  foot  of  the  cutting  edge.  As  soon  as  this  was 
done  hydraulic  pressure  was  put  upon  the  jacks,  sometimes  to  the  amount  of  5,000 
pounds  per  square  inch,  and  the  shield  forced  ahead  16  or  18  inches,  enough  for 
another  ring  of  plates,  the  working  chambers  again  being  filled  with  the  displaced 
material.  On  December  24  the  last  of  the  black  mud  was  passed  through,  and 
lying  next  to  it,  at  an  angle  of  40  degrees  towards  Long  Island,  white  decomposed 
feldspar  was  found,  containing  fragments  of  decomposed  quartz  charged  with 
sulphuretted  hydrogen.  An  important  departure  was  now  made  in  the  method  of 
erecting  the  cast-iron  lining  by  making  the  successive  rings  break  joint,  instead  of 

221 


TUNNEL    SHIELDS 

having,  as  had  previously  been  the  case  in  all  iron-lined,  tunnels,  the  horizontal 
joints  in  a  continuous  line.  On  January  16,  1894,  the  end  of  the  soft  seam 
was  reached  with  the  shield,  and  rock  was  again  entered  after  having  passed 
through  98  feet  of  soft  ground.  This  rock  resembled  slightly  the-  rock  on 
Blackwell's  Island.  It  was  in  a  much  shattered  condition,  with  many  loose  heads 
and  small,  soft  veins.  As  this  material  required  support  in  the  heading  and  a 
permanent  lining,  and  as,  in  its  present  condition,  there  was  no  assurance  that  it 
might  not  again  pass  into  soft  material,  shield  tunnelling  was  still  continued. 
Small  machine-drills  were  set  up  in  the  four  working  chambers  of  the  shield  upon 
arms  bolted  to  the  vertical  platform,  and  the  rock,  drilled  and  blasted,  just  ahead 
of  the  shield.  The  progress  of  4  feet  per  day  was  made  in  this  material  for  a  dis- 
tance of  about  65  feet,  when  solid  rock  was  met  with,  and  the  shield  was  dismantled 
and  removed,  the  remainder  of  the  work  from  the  New  York  end  of  the  tunnel 
being  carried  out  by  the  ordinary  process  of  drilling  and  blasting  the  rock.  This 
was  in  February,  1904,  and  in  the  same  month  shield  work  was  commenced  at  the 
Long  Island  end  of  the  tunnel,  where,  however,  no  special  difficulties  were  met 
with  in  passing  through  the  soft  seam  of  green  chlorite.  For  a  length  of  nearly 
90  feet  the  tunnel  was  lined  with  cast  iron  erected  under  compressed  air,  the  work 
occupying  a  month,  after  which  the  shield  was  removed,  and  the  air  pressure  taken 
off,  the  white  limestone  which  was  then  met  with  being  worked  by  ordinary  methods. 
At  the  point  where  the  tunnel  passed  from  this  material  to  the  gneiss  rock  of  the 
New  York  end,  a  further  soft  seam  of  decomposed  chlorite  was  passed  through 
which  necessitated  the  re-employment  of  compressed  air,  and  for  a  distance  of 
40  feet  the  tunnel  was  lined  with  cast  iron,  erected,  however,  without  a  shield,  i 
The  tunnel  was  completed  in  July,  1894,  two  years  having  been  occupied  in 
constructing  the  entire  length  of  2,516  feet. 


222 


Chapter    VII 

THE  SHIELD  IN  WATER-BEARING  STRATA  (continued) 

THE  VYRNWY  AQUEDUCT  TUNNEL — COMMENCED  WITHOUT  A  SHIELD,  AND  WITHOUT  COM- 
PRESSED AIR — FAILURE  OF  OPERATIONS — SHIELD  AND  COMPRESSED  AIR  PROVIDED — 
DESCRIPTION  or  SHIELD — TIMBER  SAFETY  DIAPHRAGM  OR  TRAP  IN  TUNNEL  BEHIND 
SHIELD — SECOND  ABANDONMENT  OF  THE  WORKS — RECONSTRUCTION  OF  THE  SHIELD — 
THE  "  TRAP  "  DIAPHRAGM — DOUBLE  AIRLOCK  USED  IN  THE  TUNNEL — THE  GREENWICH 
SUBWAY — DESCRIPTION  OF  TUNNEL — MACHINERY  AND  PLANT — THE  CAISSONS — THEIR 
AIRTIGHT  FLOORS — THE  PLUGS  IN  TUNNEL  OPENINGS  OF  CAISSONS — ERECTION  AND 

RlVETTING  OF  CAISSONS SINKING  OF  CAISSONS  IN  COMPRESSED  AlR — ERECTION  OF  SHIELD 

IN  CAISSONS — OPENING  OUT  THE  TUNNEL  FACE — THE  AIRLOCK  AND  BULKHEAD — SAFETY 
DIAPHRAGMS  IN  TUNNEL — RATE  OF  PROGRESS  IN  TUNNELLING — THE  SHIELD,  DETAILED 
DESCRIPTION — THE  "  TRAP  "  DIAPHRAGM — THE  FACE  RAMS — ORIGINAL  METHOD  OF 
WORKING  A  FAILURE — TRIAL  OF  NEEDLES  IN  THE  FACE — A  POLED  FACE  ADOPTED,  WITH 
CLAY  POCKETS  IN  FRONT  OF  CUTTING  EDGE— DESCRIPTION  OF  WORKING — ALTERATION  OF 
SHIELD  DIAPHRAGMS — VENTILATION  OF  SHIELD — COST  OF  SHIELD,  AND  WORKING  GANG 
REQUIRED — THE  LEA  TUNNEL — SHIELD  CHAMBER  AND  AIRLOCKS— SAFETY  DIA- 
PHRAGMS AND  VERTICAL  AIRLOCK — DETAILS  OF  SHIELD — THE  BAKER  STREET  AND 
WATERLOO  RAILWAY — SHAFTS  IN  RIVER — DETAILS  OF  SHIELD — COMBINATION  OF  HCOD 
AND  SHUTTERS — TIMBERING  OF  THE  FACE — DESCRIPTION  OF  THE  METHOD  OF  WORKING 

The  Vyrnwy  Aqueduct  Tunnel  under  the  River  Mersey  at  Fidler's  Ferry  * 

AT  the  same  time  (1888)  that  the  assisted  shield  as  described  previously  was 
employed  on  the  City  and  South  London  Railway,  another  undertaking  in- 
volving the  driving  of  a  tunnel  through  water-bearing  material  of  an  extremely 
difficult  nature  was  taken  in  hand,  in  connexion  with  the  Liverpool  water  supply 
from  Lake  Vyrnwy. 

It  was  a  necessary  part  of  this  scheme  to  construct  a  tunnel  to  carry  the  main 
pipes  of  the  aqueduct  under  the  River  Mersey  above  Liverpool,  access  to  the  tunnel, 
which  was  of  cast  iron  and  9  feet  in  internal  diameter,  being  gained  by  cast-iron 
lined  shafts  sunk  on  either  side  of  the  river. 

The  shafts  were  originally  constructed  with  a  view  to  placing  the  tunnel  about 
100  feet  below  high  water  level,  it  being  known  that  boulder  clay  was  to  be  found 
at  that  depth,  and,  it  was  hoped  by  the  engineer,  of  sufficient  tenacity  to  exclude 
the  water  beds  which  permeated  the  beds  above,  and  permit  of  the  construction  of 
the  tunnel  not  only  without  a  shield,  but  without  compressed  air. 

A  commencement  was  made  (1888)  with  the  tunnel  at  the  depth  mentioned, 
no  shield  being  used,  but  compressed  air  being  employed. 

When,  however,  only  some  60  feet  out  of  a  total  length  of  805  feet  had  been 
constructed  after  eighteen  months'  work,  the  contractor  who  had  undertaken  the 
work  withdrew. 

1  Simms'  Practical  Tunnelling,  p.  449.     Proc.  Inst.  C.E.,  vol.  cxxiii.  pp.  100-105. 

223 


TUNNEL    SHIELDS 

The  material  met  with,  instead  of  proving,  as  was  hoped,  clay  homogeneous  in 
character,  was  very  variable  and  bad  to  work  in.  In  spite  of  the  use  of  compressed 
air,  water  made  its  way  into  the  tunnel,  ultimately  completely  flooding  it. 

Another  contractor  having  undertaken  the  work,  it  was  decided,  on  the  advice 
of  Mr.  Greathead,  to  abandon  the  length  of  tunnel  already  constructed  and  to 
drive  another  one  at  a  level  of  about  50  feet  below  high  water,  and  to  do  it  under 
a  shield  in  compressed  air,  it  being  known  that  at  that  level  the  material  met  with 
would  be  water-bearing,  and  of  loose  open  character.  This  proved  only  too  correct 
a  description  of  the  material  met  with,  with  the  additional  inconvenience  that  the 
character  of  the  working  face  varied  constantly  even  in  the  length  of  excavation 
required  for  a  single  ring  of  the  tunnel  lining.  Clay,  coarse  ballast,  and  running  sand 


^ectLoru.  ttalf  Front  IE LevaJiorv. 

FIG.   145.     VYBNWY  AQUEDUCT  TUNNEL,  LIVERPOOL. 
The  Shield  as  first  constructed. 

were  all  met  in  turn,  and  frequently  the  vertical  face  of  the  tunnel  work  showed 
bands  of  them  all. 

The  compressed  air  arrangements  followed  the  usual  lines,  but  the  design  of 
the  shield  was  a  departure  from  the  type  used  at  the  Tower  Subway  and  on  the 
City  and  South  London  Railway.  The  results  obtained  with  the  shield  as  at  first 
designed  were  not  satisfactory,  but  some  of  the  features  of  the  original  shield  are 
of  interest  in  view  of  later  developments,  and  the  improvements  introduced  during 
the  progress  of  the  work  have  been  elaborated  since,  and  used  in  this  country  in 
the  Greenwich  Tunnel  under  the  Thames,  and  in  the  subaqueous  portion  of  the 
Baker  Street  and  Waterloo  Railway  in  London. 

The  shield  as  at  first  designed  is  shown  on  Fig.  145,  jthe  details  of  the  hydraulic 
rams,  pumps,  and  pipe  connexions  being  omitted  from  the  drawings. 

224 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  outside  cylinder  or  skin  consisted  of  two  f-inch  steel  plates,  made  to  break 
joint  in  the  ordinary  way,  its  inside  diameter  being  10  feet  1J  inches,  thus  having 
|  inch  play  round  the  cast-iron  lining  of  the  tunnel.  At  the  front  the  skin  was 
stiffened  by  a  steel  plate  cutting  edge.  The  total  length  over  all  was  11  feet  7  inches. 

The  front  diaphragm,  instead  ofbeing,  as  in  the  Greathead  shield,  composed 
of  a  fixed  vertical  plate  pierced  by  a  rectangular  door  in  the  centre,  was  made  of 
curved  plates,  some  of  which  were  removable,  having  their  concave  sides  to  the 
front  of  the  shield.  This  diaphragm,  f  inch  in  thickness,  was  at  its  circumference, 


JfaJfJTronkELe ycition,. 


FIG.   146.     VYKNWY  AQUEDUCT  TUNNEL,  LIVERPOOL. 
The  Shield  as  altered.     (For  Section  on  line  A,  A,  see  Fig.   147.) 

where  it  joined  the  skin,  2  feet  from  the  cutting  edge ;  at  its  centre  on  the  axis  of 
the  shield  it  was  3  feet  from  the  front. 

It  was  secured  to  the  skin  by  angle  irons,  by  gussets  A,  A,  behind,  and  by 
T  irons,  6  inches  by  3  inches  by  f  inch  in  front,  these  latter  B,  B,  serving  also  as 
supporting  brackets  to  the  cutting  edge.  (Fig.  145.) 

The  outer  portion  of  the  diaphragm  so  attached  to  the  skin  was  of  course 
irremovable,  but  the  central  portion,  about  6  feet  6  inches  in  diameter,  was  formed 
of  seven  separate  removable  plates  C,  C,  C,  joined  to  each  other  and  the  outer  portion 
of  the  diaphragm  by  cover  strips  fastened  with  f  inch  bolts.  In  the  diaphragm 
were  hand  holes  D,  D,  and  small  ones  E,  E,  through  which  a  hose  pipe  could  be 
introduced  to  loosen  the  material  in  front  by  means  of  water  jets.  In  the  lowest 

225  Q 


TUNNEL    SHIELDS 

of  the  radial  plates,  a  sluice  valve  was  fitted  so  that  the  semifluid  material  washed 
down  by  these  jets  could  be  passed  out  behind  the  shield. 

Immediately  behind  the  gussets  A,  A,  was  placed  a  circular  plate  F,  secured 
by  an  angle  iron  to  the  skin,  and  rivetted  also  to  the  gussets. 

Behind  this  plate  was  a  cast-iron  ring,  in  nine  segments,  bolted  to  each  other 
and  to  the  skin.  Each  segment  formed  a  cradle  for  a  ram,  7  inches  in  diameter, 
which  abutted  on  the  front  flange  of  the  segment. 

The  design  of  the  shield  is  open  to  criticism,  and  in  actual  work  the  combina- 
tion of  a  closed  face  and  the  use  of  water  jets  failed  to  justify  the  inventor. 


'Lates.  DO. 


Jfcdf  3ec£lorL  flfl. 


of  Box. 


rams  and  rang  -castings  removed- 


FIG.  147.     VYBNWY  AQUEDUCT  TUNNEL,  LIVERPOOL. 
The  Shield  as  altered.     (For  position  of  Section  A,  A,  see  Fig.   146.) 


But  some  features  of  the  shield  were  an  advance  on  previous  machines,  and  as 
regards  the  water- jets  it  must  be  noted  that  Mr.  Greathead  himself  at  that  time 
favoured  their  use  in  tunnel  work,  not  merely  as  subsidiary  aids,  but  as  a  principal 
method  of  working  in  loose  ground. 

He  had  in  1874,  and  later  in  1884,  taken  out  patents  for  shield  work  on  the 
same  principle. 

The  placing  of  the  front  diaphragm  of  the  shield  sufficiently  far  back  from  the 
cutting  edge  to  provide  a  working  space  in  front  of  it,  under  the  protection  of  the 
skin,  was  a  distinct  advance.  As  built,  the  projecting  skin  lacked  strength,  and 
this  was  one  of  the  causes  of  trouble. 

The  arrangement  of  the  diaphragm  by  which  a  larger  or  smaller  opening  to 

226 


THE    SHIELD    IN    WATER-BEARING    STRATA 

the  face  could  be  given  at  will  was  also  an  improvement,  and  in  the  more  modern 
plan  of  constructing  the  diaphragm  partly  in  removable  horizontal  strips  has 
proved  very  useful  in  several  cases. 

The  arrangement  of  the  diaphragm,  as  actually  constructed,  with  a  removable 
central  plate,  and  radial  ones  around  it  was  not,  however,  satisfactory. 

When  the  shield  was  first  put  in  motion,  the  method  of  working  attempted 
was  as  follows  : — 

The  sluice  in  the  lower  removable  plate  in  the  diaphragm  being  opened,  water 
jets  were  directed  on  the  face  through  the  openings  provided  for  the  purpose,  the 
idea  being  that  the  water  jets  would  loosen  the  material  of  the  face,  and  that  the 
resulting  mixture  of  gravel  and  slurry  would  flow  out  through  the  sluice  valve, 
when  it  would  be  loaded  up  by  the  miners,  who  would  thus  be  working  always  under 
the  protection  of  the  shield.  No  timbering  or  mining  work  in  front  of  the  shield 
would  indeed  have  been  necessary  if  the  water  jet  scheme  had  been  successful. 

It  is  quite  possible  that  a  tunnel  in  perfectly  homogeneous  alluvial  material 
might  be  driven  in  this  manner  ;  in  the  actual  conditions  the  effect  of  the  water 
jets  was  simply  to  wash  out  a  channel  in  front  of  each  jet  for  a  little  distance,  whence 
the  water  found  its  way  to  the  bottom  of  the  shield,  with  little  or  no  effect  en  the 
material  of  the  face. 

After  some  trial  this  method  of  working  was  abandoned,  and  excavation  was 
carried  on  by  raking  out  material  through  an  opening  at  the  bottom  of  the 
diaphragm,  made  by  removing  altogether  the  lowest  removable  radial  plate. 

Progress  was  slow,  owing  to  the  smallness  of  the  opening,  and  to  the  con- 
tinual falling  down  of  the  material  above,  as  that  at  the  level  of  the  opening  was 
removed,  which  enormously  increased  the  cubical  amount  of  excavation  necessary 
per  foot  advance  of  the  shield. 

To  lessen  this  movement  of  the  face,  needles  G  made  of  channel  bars  were 
driven  through  a  groove  cut  in  the  diaphragm  near  the  horizontal  axis.  They 
were  long  enough  to  project  some  3  feet  into  the  solid  beyond  the  cutting  edge, 
and  were  spaced  12  inches  apart.  (Fig.  145.) 

Their  use  prevented  the  falling  in  of  the  face  in  a  great  measure,  and  some 
progress  was  made. 

For  greater  security  a  timber  diaphragm  was  built  in  the  invert  of  the  tunnel 
at  a  little  distance  behind  the  shield,  its  upper  edge  being  a  few  inches  above  the 
top  of  the  bottom  opening  in  the  diaphragm  of  the  shield. 

A  water  trap  was  thus  formed,  so  that  in  the  event  of  a  "  blow  "  in  the  face, 
and  a  consequent  inrush  of  water  through  the  shield,  the  water  coming  into  the 
tunnel  would  be  held  by  the  timber  diaphragm  until  it  rose  over  the  top  of  the 
timber  by  which  time  the  opening  in  the  shield  diaphragm  would  be  under  water, 
and  the  outrush  of  air  choked. 

This  simple  and  ingenious  arrangement  was  designed  first  by  Mr.  Greathead 
in  his  shield  for  the  Woolwich  subway  in  1874  (the  shield  was  made  but  never 
used),1  and  has  been  used  since  with  entire  success  ;  indeed,  as  will  be  seen,  it  was 
the  main  feature  in  the  alterations  subsequently  made  in  the  shield  under  considera- 
tion. 

When  the  tunnel  had  been  driven  about  180  feet,  the  contractors  abandoned 
the  work,  and  the  engineer,  Mr.  Deacon,  resolved  to  complete  the  undertaking 
himself. 

1  See  Fig.   11,  page  17. 
227 


TUNNEL    SHIELDS 

Acting  on  the  advice  of  Sir  B.  Baker,  he  made  an  entire  alteration  in  the  working 
t)f  the  shield,  the  details  of  which  w&re  considerably  modified  at  the  same  time. 
Figs.  146  and  147  show  the  shield  as  altered,  or  rather  transformed,  for  it  had,  save 
the  skin  and  rams,  very  little  in  common  with  the  machine  shown  in  Fig.  145. 

In  the  original  shield  the  portion  of  the  skin  projecting  in  front  of  the  diaphragm 
and  forming  the  cutting  edge  was  not  sufficiently  stiff,  and  had  buckled  in  conse- 
quence. 

The  existing  gussets  were  strengthened  and  new  ones  added,  while  the  knife 
of  the  cutting  edge  was  provided  with  eighteen  teeth  A,  A,  projecting  6  inches  in 
advance.  The  effect  of  these  teeth  was  to  immensely  reduce  the  hydraulic  power 
required  to  move  the  shield  forward. 


FIG.   148.     VYRNWY  AQUEDUCT  TUNNEL,  LIVERPOOL. 
Details  of  Cast  Iron  Lining  of  Tunnel. 

The  principal  alteration  made,  however,  was  the  placing  of  a  water  trap  on 
the  shield  itself  instead  of  the  previous  makeshift  one  in  the  form  of  a  timber 
diaphragm  in  the  tunnel  behind  the  shield. 

To  effect  this  the  lower  half  of  the  shield  diaphragm  was  removed  up  to 
within  a  few  inches  of  the  horizontal  axis  of  the  shield,  and  provision  made  for 
closing,  if  necessary,  the  semicircular  opening  so  made  by  removable  steel  strips 
B,  B,  stiffened  with  angle  irons. 

Behind  this  diaphragm  was  fixed  a  second  one  C,  C,  or,  rather,  this  second 
one  was  a  built  frame  forming  three  sides  and  bottom  of  a  box,  of  which  the  fourth 
side  was  represented  by  the  lower  plates  in  the  front  diaphragm.  The  sides  of 
this  box  were  made  of  removable  plates  D,  D,  similar  in  construction  to  those  in 
the  front. 

228 


THE    SHIELD    IN    WATER-BEARING    STRATA 

On  the  top  of  this  box  was  fitted  a  sliding  lid  E,  which,  when  closed,  completely 
shut  up  the  face  of  the  shield. 

The  top  of  the  back  plates  of  the  trap  was  about  6  inches  above  the  bottom 
of  the  upper  solid  half  of  the  front  diaphragm.  Six  inches  was  therefore  the  amount 
of  "  water-seal,"  supposing  the  shield  to  have  no  roll  to  either  side. 

This  arrangement  may  be  described  as  an  application,  on  a  larger  scale,  of  the 
principles  of  action  which  are  shown  in  the  ordinary  drinking  fountain  in  a  bird- 
cage. 

In  this,  the  domed  reservoir  holds  a  certain  amount  of  water,  which  would 
flow  away  from  the  opening  at  the  base,  were  it  not  for  the  air  pressure  without. 
In  the  shield,  as  in  the  bird's  drinking  fountain,  the  flow  of  water  stops  immediately 
the  opening  is  "  sealed  "  by  the  water. 

In  an  ordinary  shield  and  also  in  the  "  trap  "  shield  when  the  face  is  dry,  the 
face  where  the  air  pressure  and  water  pressure  meet  is  vertical,  and  consequently 
the  pressure  of  air  necessary  to  keep  all  the  face  dry  must  equal  the  water  pressure 
at  the  invert  of  the  shield.  Immediately  a  "  blow  "  at  the  face  occurs,  and  the 


FIG.   149.     VYRNWY  AQUEDUCT  TUNNEL,  LIVEKPOOL. 
Bulkhead  with  Double  Airlock. 


"  trap  "  of  the  shield  is  closed  by  water,  the  vertical  face  becomes  a  horizontal 
one,  and  one,  too,  where  the  hydraulic  head  is  less  by  the  height  of  the  "  trap  ''* 
water  level  above  the  invert  than  the  air  pressure,  thus  insuring  the  holding  of  the 
incoming  water  by  the  air  pressure. 

The  needles  in  front  of  the  shield,  which  had  previously  been  found  serviceable, 
might,  it  was  thought,  prove  useful  again,  and  the  openings  for  them  in  the  dia- 
phragms were  retained. 

The  number  of  rams  was  increased  from  nine  to  ten,  and  consequent  on  this 
change,  an  extra  ring  of  cast-iron  segments  F,  F,  was  introduced,  partly  to  better 
distribute  the  thrust  of  the  rams,  and  partly  to  stiffen  the  skin  of  the  shield  and 
make  room  for  the  water-seal. 

The  shield  as  altered  proved  very  well  adapted  to  its  work,  the  miners  gaining 
courage  to  pass  in  front  of  the  diaphragm  and  excavate  the  face,  fixing  light  timber 
supports  stayed  from  the  cutting  edge  of  the  shield,  and  the  construction  of  the 
tunnel  made  good  progress  ;  an  average  speed  of  34  feet  per  week  was  maintained, 
the  greatest  distance  bored  in  any  one  week  being  57  feet. 

On  several  occasions  "  blows  "  occurred  in  the  face,  but  when  the  miners  had 
time  to  close  the  door  E,  no  flooding  of  the  tunnel  took  place,  and  by  increasing 

229 


TUNNEL    SHIELDS 

the  air  pressure  it  was  easy  to  partially  dry  the  material  filling  the  trap  and  remove 
it. 

When  a  blow  occurred  too  suddenly  for  any  precautions  to  be  taken,  a  certain 
amount  of  water  and  sand  entered  the  tunnel.  The  water-seal,  formed  as  soon  as 
the  water  from  the  face  rose  above  the  bottom  of  the  front  diaphragm,  was  always 
effective,  and  only  on  one  occasion  did  any  appreciable  quantity  of  material  find 
its  way  into  the  tunnel. 

The  history  of  this  shield  is  a  very  instructive  example  of  the  making  of  an 
ineffective  machine  into  an  effective  one,  and  it  is  interesting  to  compare  its  evolution 
with  that  of  the  Greenwich  Tunnel  shield  described  later  in  this  chapter,  as  the 
latter  machine  was,  as  at  first  constructed,  an  attempt  to  combine  in  one  machine 
the  good  points  of  the  first  and  second  shields  of  the  Vyrnwy  Tunnel. 

As  stated  above,  the  compressed  air  arrangements  followed  the  usual  lines, 
and  do  not  call  for  any  special  description. 

One  detail,  however,  may  be  noted. 

The  airlock  in  the  tunnel  (Fig.  149)  did  not  apparently,  as  first  made,  give 
a  sufficient  sense  of  security  to  the  miners,  and  there  was  therefore  constructed  on 
the  pressure  side  of  the  working  lock  a  second  bulkhead,  having  in  its  centre  a 
wooden  door  opening  towards  the  pressure  chamber.  The  effect  of  this  was  prac- 
tically to  double  the  capacity  of  the  lock,  but  beyond  this  there  does  not  seem  to  be 
any  advantage  in  the  way  of  additional  security,  and  the  double  lock  has  not  been 
employed  since. 

The  Greenwich  Subway  (1899) 

The  Greenwich  Subway *  consists  of  a  tunnel  under  the  River  Thames, 
-connecting  the  Isle  of  Dogs,  Poplar,  on  the  north  bank  with  Greenwich  on  the  south. 
Two  shafts  43  feet  in  external  diameter,  and  respectively  44  and  50  feet  deep  from 
.ground  level  to  the  tunnel  entrance  contain  circular  stairways  and  lifts  giving 
•access  to  the  tunnel.  These  shafts  were,  during  construction,  the  only  means  of 
access  to  the  tunnel  works  (see  Fig.  170). 

The  tunnel  itself  is  12  feet  9  inches  in  external  diameter,  and  is  lined  with 
cast  iron  (see  Figs.  30,  31,  32). 

From  each  shaft  the  tunnel  dips  towards  the  centre  of  the  river  with  a  gradient 
of  1  in  15  ;  the  middle  portion  of  the  tunnel  is  on  a  gradient  of  1  in  227,  falling 
towards  the  Greenwich  shore.  These  gradients  were  entailed  by  the  advisability, 
from  motives  of  economy,  of  limiting  the  depths  of  the  shafts,  and  by  the  necessity 
of  complying  with  the  conditions  of  the  Act  authorizing  the  undertaking,  which 
stipulated  that  the  level  of  the  tunnel  should  be  such  as  to  allow  of  dredging  in  the 
ri^er  a  channel  500  feet  wide  and  48  feet  deep  at  high  water. 

It  was  arranged  to  start  the  tunnel  from  the  northern,  or  Poplar  shaft,  and  the 
contractors'  yard  and  general  machinery  and  workshops  were  laid  out,  therefore, 
at  this  end.  The  machinery  installed  for  the  work  was  as  follows,  in  addition  to 
the  ordinary  hoisting  machinery  in  the  shaft.  There  were  two  large  air  com- 
pressors built  by  Walkers  of  Wigan  with  a  capacity,  estimating  the  efficiency  at 
80  per  cent.,  of  52,000  cubic  feet  per  hour  each  at  any  reasonable  pressure. 

These  engines  worked  with  a  boiler  pressure  of  100  pounds,  and  the  high  and 
low  pressure  cylinders  were  respectively  18  inches  and  24  inches  diameter,  the 

1  Proc.  Inst.  C.E.,  vol.  cl.     The  Author's  Paper  on  The  Greenwich  Footway  Tunnel. 

230 


THE    SHIELD    IN    WATER-BEARING    STRATA 


length  of  stroke  being  42  in?hes. 
The  air  cylinders  were  24  inches  in 
diameter. 

These  were  for  the  supply  of 
air  to  the  working  chambers  in  the 
shafts  and  tunnel  only,  and  in  ad- 
dition an  Ingersoll-Sargeant  straight 
line  piston  inlet  compressor  of  40 
I.H.P.  with  an  automatic  regulator 
and  unloading  arrangement  was  em- 
ployed, for  working  the  pneumatic 
rivetters  which  were  largely  used 
in  putting  together  the  shafts,  for 
the  blast  in  rivet  forges,  and  for 
driving  sundry  small  machines  in 
the  workshops,  for  driving  the  haul- 
ing engine  within  the  tunnel,  and 
later  for  pumping. 

Two  small  steam-driven  hy- 
draulic compressors  were  employed 
for  supplying  power  to  the  shield 
rams.  These  could  work  up  to  a 
pressure  of  3  tons  per  square  inch. 
The  steam  cylinders  were  8  inches 
in  diameter  with  10  inches  stroke, 
the  water  cylinders  being  1  inch  in 
diameter.  They  were  fitted  with 
an  automatic  unloading  device  to 
cut  off  the  steam  when  the  hydraulic 
pressure  rose  above  the  required 
pressure. 

Dynamos  driven  by  small  ver- 
tical engines  were  provided  for 
lighting  the  tunnel  and  works. 

A  wash-out  pump  was  provided 
with  the  intention  of  using  water 
from  the  River  Thames  to  wash 
down  the  working  face  of  the  tun- 
nel, but  was  not  used.  It  was, 
however,  employed  several  times  for 
forcing  water  outside  the  shafts 
when  sinking,  with  a  view  to  lessen 
the  skin  friction. 

In  addition  to  the  ordinary 
offices,  stores,  cement  shed,  and 
workshops,  a  set  of  rooms,  properly 
heated,  with  proper  washing  appli- 
ances, were  provided  for  the  gangs 
working  in  compressed  air. 


~il    £  > 

lil 


— 1 


W   .-fi 

«    Sb 
0   § 


231 


TUNNEL    SHIELDS 

A  medical  lock  and  medical  officer's  room  were  of  course  required. 

The  general  arrangement  of  the  contractors'  yard  is  shown  in  Fig.  151. 

The  two  shafts  are  alike  in  general  construction,  and  differ  only  in  depth,  that 
on  the  Poplar  shore  measuring  60  feet  from  top  to  cutting  edge,  and  the  Greenwich 
shaft  66  feet  7  inches. 

The  caissons  (see  Figs.  152  and  153)  are  35  feet  in  internal,  and  43  feet 
in  external  diameter,  and  are  formed  with  two  steel  skins,  the  4-foot  space 
between  which  is  filled  with  6  to  1  Portland-cement  concrete.  The  skins  are  formed 
of  horizontal  rings  built  up  of  plates,  which  are  generally  about  4  feet  9  inches  in 
depth  and  vary  in  thickness  from  f  inch  at  the  bottom  of  the  caisson  to  T\  inch  at 
the  top  ;  the  vertical  joints  in  each  ring  are  arranged,  wherever  possible,  to  break 


c 

J        jo*  3^ 

In\actoJ>    •*« 

Ucat 

Omcer 

C  *    m.  f    n.   t          3   t    t>    i-   a 

^.^^ 

FIG.   151.     GREENWICH  TUNNEL,  LONDON. 
Contractor's  Yard. 


joint  with  the  rings  above  and  below.  The  skins  are  braced  together  with  vertical 
angle-bar  frames  and  horizontal  angle-bracings.  At  the  points  where  the  main 
girders  of  the  air-tight  floors  are  attached,  the  webs  of  which  are  carried  through 
to  the  outer  skin,  vertical  plate  gussets  are  fixed  in  the  rings  above  and  below,  to 
resist  deformation  under  the  severe  stress  produced  at  the  ends  of  the  girders  by 
the  air-pressure  on  the  floor.  Around  the  tunnel-opening,  also,  the  caisson  is 
strengthened  by  numerous  plate  gussets. 

The  cutting  edge  is  7  feet  in  depth,  and  is  formed  by  inclining  the  inner  skin 
to  meet  the  outer  :  at  the  edge  thus  formed  an  extra  plate,  |  inch  thick,  is  placed 
round  the  outer  skin  for  greater  stiffness.  The  bottom  edges  of  the  cutting  edge 
plates  are  not  flush  with  one  another,  but  each  edge  is  1  inch  above  that  of  the 

232 


THE    SHIELD    IN    WATER-BEARING    STRATA 

plate  outside  it,  so  as  to  allow  of  easy  caulking.  All  caulking  at  this  and  other 
parts  of  the  shafts  was  done  by  means  of  pneumatic  tools.  The  cutting  edge  is 
further  strengthened  by  forty-eight  triangular  vertical  gussets  \  inch  thick.  At 
the  top  of  the  inside  skin  of  the  cutting  edge  is  fixed  round  the  entire  circumference 


FIG.   152.     GREENWICH  TUNNEL,  LONDON. 
Method  of  sinking  Shafts. 

an  angle-bar  A  (Fig.  152),  6  inches  by  6  inches  by  £  inch.  It  was  intended  to  use 
this  as  a  bracket  under  which  timbers  could  be  fixed  in  order  to  control  the  sinking 
of  the  shaft.  In  practice,  however,  all  such  work  was  done  by  timbers  fixed  under 
tho  main  girders  of  the  air-tight  floor. 

233 


TUNNEL    SHIELDS 


The  outer  skin  of  the  caisson  was  without  the  usual  batter,  but  the  plates  of 
each  ring  were  inclined  outwards  towards  the  top,  so  that  the  top  edge  of  every 
ring  touched  a  vertical  cylinder.  The  caisson  was  therefore  of  uniform  diameter 
from  top  to  bottom,  excepting  for  the  extra  f-inch  plate  at  the  cutting  edge  ;  and 


'•'•16  to 2  FC.doricre£0:S  ^V* 


FIG.   153.     GREENWICH  TUNNEL,  LONDON. 
Method  of  removing  "  Plug  "  in  Shaft  to  commence  Tunnelling. 

although  the  absolute  necessity  of  preventing  any  spreading  during  the  erection 
of  the  successive  rings  (whereby  the  caisson  would  have  been  made  wider  at  the 
top  than  at  the  bottom)  rendered  great  care  necessary  in  rivetting  up  the  work,  still 
the  extra  trouble  and  expense  so  incurred  were  more  than  compensated  for  by 
the  results  obtained  in  the  sinking  of  the  shafts  :  results  which  the  Author  believes 
were  largely  due  to  the  absence  of  taper  in  the  shafts  and  of  consequent  settling 

234 


THE    SHIELD    IN    WATER-BEARING    STRATA 

of  the  ground  around  them.  Iron  rivets  were  used  throughout ;  and  all  rivetting, 
whether  gap-rivetters  or  hand-pistols  were  used,  was  done  by  compressed  air  power. 
Three  tiers  of  pipes  were  fitted  through  the  sides  of  the  caisson,  and  connected  to 
the  wash-out  pump,  through  which  water  could  be  pumped  to  lubricate  the  outer 
skin  in  case  of  necessity.  These,  however,  were  little  used. 

Each  caisson  is  provided  with  two  air-tight  floors,  a  permanent  one  B  fixed 
below  the  tunnel-opening  and  above  the  cutting  edge,  and  a  provisional  one  C 
fixed,  when  in  use,  a  few  feet  above  the  tunnel-opening.  These  floors  were  alike 
in  construction,  being  formed  of  two  pairs  of  main  girders  crossing  each  other  at 
right  angles  ;  the  intermediate  areas  were  crossed  by  small  girders  and  the  whole 
frame  was  covered  by  buckled  floor-plates  bolted  to  the  girders.  The  provision 
of  two  air-tight  floors  has  the  advantage  that,  while  a  single  floor  above  the  tunnel- 
opening  is  a  necessity  for  starting  the  tunnel-shield  in  water-bearing  strata,  the  use 
of  the  lower  floor  only  during  the  operation  of  sinking  the  caisson,  while  allowing 
an  ample  working-chamber  underneath  for  the  miners,  enables  any  kentledge  used 
in  sinking  to  be  placed  near  the  bottom  of  the  caisson,  and  so  keeps  the  centre  of 
gravity  of  the  whole  structure,  when  sinking,  much  lower  than  is  possible  with  an 
upper  floor  only.  This  lessens  materially  the  tendency  of  the  shaft  to  tilt  over  to 
one  side  when  sinking.  Against  this  advantage  must  be  set  the  increased  cost  of 
the  shafts,  due  to  the  extra  depth  required  to  make  room  for  a  floor  with  girders 
4  feet  6  inches  in  depth,  clear  of  the  tunnel-opening  above  it  and  the  cutting  edge 
below  (see  also  page  185). 

The  opening  in  each  shaft  for  the  tunnel  is  15  feet  3  inches  in  diameter.  Dur- 
ing the  sinking  of  the  caisson  it  was  closed  by  a  "  plug  "  H  formed  of  steel  plates 
(see  Figs.  154,  155  and  156),  fitted  between  girders  in  such  a  way  as  to  be  removable 
singly  when  the  shaft  was  sunk  to  its  proper  depth  and  the  shield  was  ready  to 
start.  The  face-plates  of  the  "  plug  "  were  held  in  place  by  two  girders,  which 
divided  the  opening  into  three  horizontal  strips  about  5  feet  deep.  These  areas 
were  again  divided  vertically  by  three  smaller  girders,  5  feet  apart.  The  face-plates 
were  fitted  and  bolted  to  the  girders  in  the  areas  so  formed,  wood  packings  being  used 
to  facilitate  their  removal  when  necessary.  Bolts  were  also  used  for  making  up 
the  girders,  so  that  they  could  be  removed  in  pieces.  The  girders  were  bolted  to 
horizontal  brackets,  fixed  on  the  skin  of  the  circular  plug-opening,  and  themselves 
removable. 

For  convenience  of  removal,  the  plug  was  made  slightly  smaller  in  diameter 
than  the  opening  which  it  was  required  to  fill.  It  was  intended,  when  the  plug 
was  in  position,  to  drive  hardwood  wedges  between  it  and  the  shaft  plates  so  as  to 
make  a  water-tight  joint.  The  contractors,  however,  preferred  to  caulk  with  soft 
wood,  subsequently  driving  into  this  thin  hardwood  wedges,  a  method  of  working 
which  proved  very  successful. 

The  erection  and  sinking  of  the  caissons  was  carried  out  as  follows,  the  same 
method  being  adopted  in  each  shaft  : — 

All  the  steelwork  used  in  the  caisson  was  first  put  together  in  the  builders' 
yard  in  sections  of  not  less  than  four  rings  at  a  time,  so  as  to  ensure  good  fitting  ; 
and  so  well  was  this  work  performed  that,  when  it  was  put  together  again  on  the 
site  of  the  shaft,  but  little  adjustment  was  found  necessary.  To  ensure  the  caisson 
being  perfectly  cylindrical,  which  was  particularly  important  for  the  reason  already 
mentioned,  the  contractors  were  required  to  put  in  the  concrete  filling  between 
the  skins  as  the  erection  of  the  steelwork  advanced;  that  is  to  say,  the  concrete  filling 

235 


TUNNEL    SHIELDS 

for  each  ring  was  (save  in  exceptional  circumstances,  such  as  frosty  weather)  always 
put  in  to  within  2  feet  of  the  top  before  the  rivetting  of  the  next  ring  was  proceeded 
with.  It  was  therefore  necessary  to  make  each  ring  absolutely  correct  before  going 
on  with  the  next  above,  because,  the  concrete  filling  once  in,  no  further  adjustment 
was  possible.  Only  in  the  two  top  rings,  when  all  danger  of  the  shaft  being  jammed 


I'dLa.. 


FIG.  154.  GREENWICH  TUNNEL,  LONDON. 

Framing  or  "  plug  "  closing  Tunnel  opening  during  operation  of  sinking  Shaft.     (For  cross  sections 

A,  A,  and  B,  B,  see  Fig.  155.) 


in  sinking  had  disappeared,  was  this  procedure  departed  from.  This  method  of 
working  was  also  advantageous  in  ensuring  that  the  concrete  filling  was  tipped  only 
a  few  feet  from  the  skip.  As  soon  as  the  cutting  edge  and  the  lower  air-tight  floor 
with  the  ring  surrounding  it  were  rivetted  up,  and  the  concrete  filling  was  in  place, 
the  blocks  on  which  the  caisson  had  stood  were  removed  and  the  sinking  was  com- 
menced. 

The  erection  of  the  northern  or  Poplar  caisson  was  commenced  on  September 

236 


THE    SHIELD    IN    WATER-BEARING    STRATA 

26,  1899,  and  by  March  13,  1900,  one-half  was  erected  and  the  cutting  edge  was 
sunk  to  a  depth  of  24  feet  6  inches  below  ground  level,  when  the  quantity  of  water 
met  with  prevented  further  progress,  and  sinking  was  stopped  until  the  arrange- 
ments for  working  in  compressed  air  could  be  completed.  So  far  the  sinking  of  the 
caisson  had  been  done  in  the  open,  the  material  inside  the  shaft  being  removed 
in  the  ordinary  way,  and  the  lowering  of  the  structure  being  controlled  by  foot-blocks 
under  the  cutting  edge.  No  kentledge  was  required  at  this  stage,  the  shaft  going 
down  easily  when  released,  and  being  never  more  than  a  few  inches  out  of  level. 
The  material  passed  through  was  mainly  river-mud  and  silty  clay  ;  but  at  22  feet 
below  the  surface  ballast  was  found,  through  which,  as  already  stated,  water  found 
its  way  from  the  river,  the  water  in  the  caisson  rising  and  falling  with  the  tide. 
Up  to  this  time  the  girders  only  of  the  lower  floor  B  had  been  fixed  in  place, 
the  floor  plates  being  left  off  so  as  to  allow  of  the  removal  of  the  material  excavated, 


Onoltxs  boLted  to  flange ,  &  rufette 
toireb. 


o   o  o  o  o  o 


' Holts  ~ 


FIG.   155.     GREENWICH  TUNNEL,  LONDON. 
Details  of  Plug.     Seotionsjan  lines  A,  A,  and  B,  B,  Fig.  154. 


while  the  girders  of  the  upper  floor  C,  save  for  the  ends  of  the  webs,  which  were 
built  into  the  caisson,  were  not  put  in.  When  the  water  became  too  deep  to  con- 
tinue work  in  the  open,  the  bottom  floor-plates  were  fixed  on  the  lower  girders, 
and  the  upper  floor-girders  were  erected  in  position  (see  Fig.  152). 

On  these  upper  girders  were  fixed  two  airlocks  D,  D,  the  one  for  men,  the 
other  for  material,  from  which  shafts  E,  E,  3  feet  6  inches  in  diameter,1  were  carried 
down  through  the  lower  floor,  thus  giving  access  to  the  working-chamber  below  it. 
On  May  2,  1900,  the  work  of  sinking  the  caisson  was  resumed  under  compressed 
air,  the  erection  of  the  steelwork  of  the  caisson  having  in  the  meantime  been  com- 
pleted to  within  15  feet  of  the  total  height.  The  remaining  rings  were  erected 
during  the  time  the  shaft  was  sinking. 

xFor  details  of  the  material  lock,  see  Figs.  131,  132,  and  133. 

237 


TUNNEL    SHIELDS 

The  method  of  sinking  the  caissons  under  compressed  air  was  as  follows. 
Starting  usually  with  the  ground-level  in  the  pressure-  or  working-chamber  about 
1  foot  6  inches  above  the  bottom  of  the  cutting  edge,  the  miners  excavated  daily 
to  a  depth  of  about  1  foot  below  it, leaving  all  round  the  cutting  edge  a  berme  between 
1  foot  and  2  feet  in  width.  The  amount  of  excavation  was  limited  by  the  number 
of  skips  which  could  be  passed  through  the  vertical  airlock  in  a  given  time  ;  and 
in  practice  it  was  found  that  four  men  working  in  the  pressure-chamber  could  keep 
the  skips  filled  as  fast  as  they  could  be  returned  empty.  Daily,  generally  during 
the  breakfast-hour  of  the  morning  shift,  when  the  men  were  out  of  the  caisson, 
the  air  pressure  in  the  working-chamber  was  lowered  until  the  caisson  commenced 
to  move.  No  further  reduction  of  pressure  was  then  made,  and  the  movement 
of  the  caisson  was  gradually  stopped  by  the  tapered  cutting  edge  burying  itself 
anew  to  a  depth  of  1  foot  to  2  feet  below  the  general  level  of  the  excavation.  The 
air  pressure  was  then  again  raised  sufficiently  to  dry  the  bottom  of  the  shaft,  and 
the  same  process  of  excavating  and  sinking  was  repeated.  It  was  found  possible 
to  carry  on  simultaneously  the  different  operations  of  plating  and  rivetting  the 
skins,  putting  in  the  concrete  filling,  and  sinking  the  shaft  ;  and  by  careful  organi- 
zation it  rarely  happened  that  one  operation  interfered  with  or  delayed  another. 
It  will  be  seen  from  the  Table  giving  details  of  the  sinking  operations  that  the  daily 
progress  under  compressed  air  was  singularly  uniform. 

The  strata  met  with  3' 00  feet  below  Ordnance  datum,  or  about  20  feet  below 
ground-level,  were  water-bearing.  The  top  surface  of  the  ballast,  which  was  found 
on  the  north  side  of  the  caisson  about  22  feet  below  ground  level,  sloped  rapidly 
southward,  and  some  difficulty  was  expected  in  keeping  the  cutting  edge  level  on 
that  account.  As  a  matter  of  fact,  for  a  few  days  the  error  of  level  of  the  caisson 
amounted  to  6  J  inches,  being  the  largest  amount  of  tilt  recorded  ;  but  this  was  put 
right,  and  subsequently  the  caisson,  though  sinking  at  an  average  rate  of  1  foot 
3  inches  per  day,  was  never  more  than  If  inch  out  of  level.  Below  the  ballast,  at 
about  41  feet  below  ground-level,  was  found  a  bed  of  close  grey  sand,  at  times 
almost  as  tough  as  soft  sandstone.  It  was  noticed  that  the  skin-friction  of  the 
caisson,  which,  when  the  lower  portion  was  in  the  ballast,  had  been  generally  4' 5 
cwt.  to  4'7  cwt.  per  square  foot,  became  less  as  the  cutting  edge  sank  deeper  into  the 
sand  ;  and  the  last  observations,  taken  when  the  caisson  was  nearly  down  to  the 
required  level,  gave  a  skin-friction  of  just  under  3-8  cwt.  per  square  foot.  The 
probable  explanation  is  that,  owing  to  the  consistency  of  the  sand,  most  of  the  air 
escaping  from  the  pressure-chamber  passed  up  close  to  the  outside  of  the  caisson 
and  so  made  an  air  lubricant  for  it.  The  lubricating-pipes  which  were  provided 
in  the  caisson  were  used  on  a  few  occasions,  but  without  appreciable  results, 
perhaps  owing  to  the  choking  of  the  pipes  by  the  dirty  water  used,  but  more  prob- 
ably owing  to  the  air  that  escaped  from  below  driving  away  the  water  from  the 
caisson  as  soon  as  it  left  the  pipes. 

The  total  weight  of  kentledge  put  into  the  caisson  was  only  921  tons,  which, 
with  the  weight  of  the  steel  and  concrete  in  the  structure,  made  finally  a  total 
weight  of  2,560  tons.  By  the  provision  of  an  air-tight  floor  at  the  bottom  of  the 
shaft,  the  contractors  were  enabled  to  provide  the  necessary  kentledge  by  tipping 
the  ballast  as  it  came  up  through  the  airlocks  into  the  shaft  below,  and  by  running 
in  water  from  a  pipe  above.  As  stated  above,  the  sinking  of  the  Poplar  shaft 
under  compressed  air  was  commenced  on  May  2,  1900.  .  It  was  sunk  to  the  required 
depth  by  the  31st  of  the  same  month.  The  Table  shows  the  daily  progress  from 

238 


THE    SHIELD    IN    WATER-BEARING    STRATA 


May  10  onward,  with  the  loads  on  the  caisson,  pressure,  etc.,  from  the  commence- 
ment of  work  under  compressed  air  until  the  shaft  was  sunk  to  the  required  level. 
TABLE  SHOWING  RATE  OF  SINKING  OF  THE  POPLAR  SHAFT. 


Date. 

Depth  of 
Cutting 
Edge 
below 
Surface. 

Cutting 
Edge 
out  of 
Level. 

Depth 

sunk  each 
time  of 
lowering. 

Load  on 
Shaft, 
including 
Kent- 
ledge. 

Air- 
Pressure. 

Remarks. 

1900. 
May. 

Feet. 

Inch. 

Feet. 

Tons. 

Lbs.   per 
Sq.  Inch. 

2 

24-80 

1 

— 

1,346 

6 

i  Pressure  put  on.     Cutting  edge  in 
i      ballast  at  north  side. 

7 

27-50 

6£ 

2-70 

1,362 

6 

Skin-friction  4  cwt.  per  foot. 

10 

28-60 

If 

1-10 

1,412 

6 

11 

29-60 

1| 

1-00 

1,457 

8 

12 

31-83 

I 

2-23 

1,618 

8 

13 

33-70 

If 

1-87 

1,826 

8 

Skin-friction  4-7  cwt.  per  foot. 

14 

35-80 

If 

2-10 

1,943 

8-10 

15 

37-26 

I 

1-46 

2,114 

10 

Skin-friction  4-75  cwt.  per  foot. 

16 

39-16 

f 

1-90 

2,257 

10-11 

17 

39-96 

f 

0-80 

2,371 

11-12 

18 

41-50 

1 

1-54 

2,371 

11-12 

Cutting  edge  in  close  grey  sand. 

19 

42-50 

1 

1-00 

2,371 

10-11 

Skin-friction  4-6  cwt.  per  foot. 

20 

43-60 

1 

MO 

2,371 

10 

21 

44-75 

i 

1-15 

2,421 

10 

j 

22 

46-42 

H 

1-67 

2,421 

10^-13 

23 

48-31 

1 

1-89 

2,421' 

11-13 

Lubricating-pipes  in  use. 

24 

49-46 

i 

1-15 

2,421 

13 

25 

49-82 

! 

0-36 

2,421 

14 

26 

51-80 

1 

1-98 

2,449 

15 

27 

53-35 

1-55 

2,477 

15| 

. 

28 

55-20 

j 

1-85 

2,505 

16 

29 

57-13 

1-93 

2,533 

161 

30 

58-55 

1 

1-42 

2,561 

16 

31 

60-30 

1-75 

2,561 

20 

Skin-friction  3-8  cwt.  per  foot. 

It  will  be  seen  from  this  Table  that  from  May  10  to  31,  a  period  of  twenty- 
two  days,  the  depth  sunk  daily  varied  but  little,  only  twice  falling  below  1  foot 
and  twice  exceeding  2  feet.  When  sunk  to  the  proper  depth  the  caisson  was  ex- 
actly plumb.  As  the  Greenwich  caisson  was  sunk  in  a  similar  manner,  and  with 
the  same  uniform  speed  and  accuracy,  the  use  of  vertical  sides  to  the  caisson,  and 
of  an  air-tight  floor  near  to  the  cutting  edge  during  sinking,  would  appear  to  be 
justified.  The  vertical  sides  of  the  caisson  prevented,  as  far  as  could  be  seen,  any 
disturbance  of  the  surrounding  ground  during  the  operation  of  sinking  ;  at  least 
no  cracks  were  visible  on  the  surface  more  than  2  or  3  feet  away,  and  those  which 
appeared  within  that  distance  were  slight.  When  the  plug  of  the  tunnel-opening 
was  removed,  and  the  shield  was  pushed  forward  into  the  solid  ground,  no  previous 
movement  of  the  strata  was  perceptible.  The  steadiness  of  the  caisson  when  in 
motion  was  no  doubt  due  in  part  to  the  slight  disturbance  of  the  surrounding 
ground  ;  but  the  lowness  of  the  centre  of  gravity  of  the  structure,  due  to  placing 
the  kentledge  near  the  bottom  of  the  shaft,  probably  also  accounted  largely  for 
the  ease  with  which  it  was  controlled. 

From  the  experience  gained  in  recent  years  it  appears  probable  that  in  sinking 
a  shaft  or  caisson  through  different  beds,  of  varying  resistance,  it  is  best  to  make 
the  caisson  with  an  uniform  or  nearly  uniform  external  diameter.  Perhaps  for 
shafts  from  50  to  100  feet  in  depth,  the  most  satisfactory  arrangement  would  be 

239 


TUNNEL    SHIELDS 

to  have  a  cutting  edge  of  about  7  feet  in  length  of  one  diameter,  and  then  above 
that  length  to  make  the  remainder  of  the  shaft  1  inch  less  in  diameter.  But  there 
is  little  doubt  but  that  a  caisson  should  have  its  sides  practically  parallel. 

As  soon  as  the  Poplar  shaft  had  been  sunk  to  the  required  depth,  the  working- 
chamber  under  the  air-tight  floor  B  was  filled  with  6  to  1  Portland-cement  concrete 
to  the  level  of  the  underside  of  the  floor-girders  ;  and  when  that  had  set,  pressure 
was  taken  off,  and  the  floor-plates  were  removed.  Concrete  was  then  filled  in 
between  the  girders  to  just  below  the  floor-level,  and  the  plates  were  replaced, 
grout  being  afterwards  forced  beneath  them  under  pressure,  through  holes  pro- 
vided for  the  purpose.  The  floor  thus  made  has  proved  to  be  absolutely  water- 
tight, no  leakage  having  been  observed  in  the  three  years  during  which  the  tunnel 
has  been  open  to  traffic.  The  material  for  the  shield  was  then  lowered  through 
the  upper  floor-girders,  the  airlocks  and  tubes  being  moved  for  the  purpose,  and 
the  shield  was  erected  on  a  timber  cradle  F  laid  on  the  lower  floor.  (The  position  of 
the  shield  during  erection  is  shown  in  Fig.  153.)  A  ring  of  concrete  G  was  then  built 
inside  the  tunnel-hood,  leaving  an  opening  large  enough  to  admit  of  the  passage  of 
the  shield.  Behind  the  shield  some  temporary  tunnel-rings  were  built,  extending 
to  the  skin  of  the  caisson,  for  the  shield  to  push  against.  This  done,  the  upper 
floor-plates  were  fixed  on  the  girders  already  in  position,  the  airlocks  were  replaced, 
and  air  pressure  was  again  applied.  The  actual  removal  of  the  "  plug  "  H  was 
carried  out  as  follows.  Across  the  plug-opening,  at  a  distance  of  4  feet  from 
the  steel  plug,  and  about  5  feet  from  the  invert  of  the  plug-opening,  a  12  inch  by 
12  inch  timber  J  was  fixed,  bearing  on  the  concrete  ring  previously  referred  to, 
which  was  itself  secured  by  rails  bolted  to  the  circular  hood.  Below  it  was  placed 
a  12  inch  by  6  inch  timber  J1,  similarly  bearing  against  the  concrete  ;  and  close 
poling  was  then  built  in  front  of  the  joists,  forming  a  diaphragm  from  the  invert 
of  the  plug-opening  to  above  the  level  of  the  lower  girder  of  the  plug.  The  lower 
plates  of  the  plug  were  then  removed  one  by  one,  and  the  space  between  the  solid 
ground  so  exposed  and  the  timber  diaphragm  was  filled  with  well-plugged  clay  K. 
The  lower  girder  was  then  removed,  the  bottom  edges  of  the  plates  above  it  being 
held  by  stretchers  to  the  12  inch  by  12  inch  timber.  Two  similar  timbers  were 
then  fixed  above  the  first,  and  close  poling  was  placed  in  front  of  them,  thus  raising 
the  timber  diaphragm  to  the  level  of  the  second  girder  of  the  plug  ;  the  plates  below 
were  in  turn  removed,  and  the  exposed  face  was  supported  by  clay  filling  as  before. 
In  this  way  all  the  steel  plug  was  removed,  and  in  its  place  the  tunnel-opening  was 
closed  by  a  timber  diaphragm  and  some  4  feet  of  clay  in  front  of  it.  The  shield 
was  pushed  forward  until  it  touched  the  timber  diaphragm,  which  was  subsequently 
cut  out  to  allow  of  its  passage. 

The  cast-iron  lining  of  the  tunnel  has  been  already  described  in  chapter  III. 
The  type  of  joint  used  proved  entirely  satisfactory,  and  after  being  three  years 
in  charge  of  the  completed  tunnel  the  Author  can  state  that  the  leakage  observed 
is  the  smallest  on  record  for  a  subaqueous  tunnel  of  this  length. 

The  whole  of  the  tunnelling  was  done  under  compressed  air.  The  vertical 
airlocks  in  the  Poplar  shaft  continued  in  use  until  142  feet  of  the  tunnel  had  been 
constructed,  and  then  a  bulkhead,  with  a  horizontal  lock  and  an  emergency-lock 
above  it,  was  built  in  the  tunnel  close  to  the  shaft,  and  the  upper  air-tight  floor  and 
locks  were  removed  and  transferred  to  the  Greenwich  shaft. 

The  bulkhead  consisted  of  two  thicknesses  of  brickwork  in  cement  separated 
by  a  cement  grouting  of  9  inches ;  the  total  thickness  being  8  feet  3  inches.  On 

240 


\v- 


^ 
^ 


tt\  < 


— a 


241 


TUNNEL    SHIELDS 

the  inner  or  pressure  side  of  the  bulkhead,  a  coating  of  neat  cement,  1  inch  thick, 
was  laid. 

The  use  of  brick  rather  than  concrete  for  bulkheads  is  preferable  by  reason  of 
the  greater  ease  with  which  the  latter  can  be  removed  when  the  bulkhead  is  no 
longer  required. 

Figs.  155  and  156  show  the  arrangement  of  the  locks  and  pipes  in  the  bulk- 
head. The  main  or  working  lock  was  supplemented  by  a  second  or  emergency 
lock,  built  in  to  the  bulkhead  above  the  other,  so  that  in  the  event  of  the  lower 
part  of  the  tunnel  becoming  flooded,  a  means  of  egress  would  be  left  for  the  miners. 


5  Was 


FIG.   156A.     GREENWICH  TUNNEL,  LONDON. 
Bulkhead  and  Airlocks  :    Outside  Elevation. 

The  arrangement  of  the  pipes  built  in  the  bulkhead  is  a  fair  example  of  the 
requirements  of  a  work  of  this  kind.  For  the  passage  into  the  air  chamber  of 
rails,  timbers,  pipes  and  generally  of  material  of  small  bulk  in  proportion  to  length, 
a  material  lock,  18  feet  long,  and  6  inches  in  diameter,  was  provided.  This  lock 
was  simply  a  flanged  pipe,  having  at  either  side  of  the  bulkhead  a  sliding  air-tight 
valve. 

The  main  air  inlet  was  12  inches  in  diameter,  and  terminated,  on  the  inner 
or  pressure  side  of  the  bulkhead,  in  a  flap  valve.  (To  this  pipe  was  afterwards 
attached  the  purifying  apparatus  described  in  chapter  II.)  This  pipe  received 

242 


THE    SHIELD    IN    WATER-BEARING    STRATA 


the  compressed  air  from  the  compressors,  the  cooling  reservoir  or  tank  being  fixed 
about  midway  between  the  compressors  and  the  tunnel.  The  fact  that  this  inlet 
pipe  delivered  the  air  immediately  inside  the  bulkhead  ensured  that,  as  the  waste 
air  pipe  and  the  various  "  blow-out  "  pipes  were  carried  forward  as  the  shield,  the 
air  of  the  whole  length  of  the  tunnel  was  continually  renewed. 

The  waste  air  pipe,  which  was  always  carried  forward  with  the  shield,  had  its 
outside  end  carried  sufficiently  far  from  the  lock  to  prevent  the  noise  of  the  escap- 
ing air  interfering  with  the  secure  working  of  the  lock,  and  the  other  exhaust  pipes 
known  as  "  blow-out  "  pipes  discharged  into  the  well  of  the  shaft,  whence  the  water 
was  pumped  by  a  Smith  Vaile  force  pump  to  the  surface  of  the  ground. 

An  air  pipe  3  inches  in  diameter  conveyed  air  at  a  pressure  of  about  100  pounds 
to  the  face  for  working  the  grouting  machine,  and  also  the  winding  engine  fixed 
in  the  roof  of  the  tunnel  immediately  within  the  bulkhead. 

For  the  working  of  the  lock  1  inch  valves  for  passing  workmen,  and  3  inch  valves 
for  material  were  used. 

The  bulkhead  was  placed  in  the  very  entrance  of  the  tunnel,  close  to  the  Poplar 
shaft,  in  which  a  lift  was  fixed,  so  that  the  miners,  in  leaving  the  pressure  chamber, 
were  conveyed  without  delay  or  exertion  on  their  part  to  the  surface. 

Safety  diaphragms  of  the  character  figured  on  page  260,  were  used  in  this 
tunnel,  and  one  was  fixed  at  the  foot  of  each  of  the  gradients  from  the  shafts  as 
soon  as  the  shield  commenced  the  ascent  on  the  Greenwich  side.  It  is  clear  from 
an  inspection  of  the  longitudinal  section  of  the  tunnel  that,  in  case  of  a  "  blow  " 
when  the  shield  was,  say,  halfway  up  the  incline,  the  working  pressure  at  the  face 
being  less  than  that  required  at  the  lower  part  of  the  tunnel,  there  would  be  no 
escape  for  the  men  at  the  face,  unless  a  refuge  was  provided  for  them  much  nearer 
than  the  main  airlock. 

Such  a  refuge  was  provided  by  the  two  diaphragms  in  the  positions  shown  in 
Fig.  150,  as  they  formed,  with  the  roof  of  the  tunnel,  an  air  space,  into  which  the 
miners  could,  on  an  emergency,  climb,  until  help  could  come  from  outside. 

With  the  object  of  removing  the  water  in  the  lower  part  of  the  tunnel  in  case 
of  a  blow  of  this  character,  two  "  blow-out  "  pipes  were  provided  at  Greenwich, 
the  one  being  carried  forward  as  usual  with  the  shield,  the  other  being  stopped 
when  its  aperture  had  reached  the  lowest  part  of  the  tunnel,  so  that  whenever 
any  water  entered  the*tunnel,  an  exit  was  provided  for  it  at  the  lowest  point  of  the 
invert. 

The  time  occupied  in  driving  the  tunnel  was  from  September  19,  1900,  to 
May  31,  1901,  or  about  eight  and  a  half  months,  the  monthly  progress  being  as 
under  : — 

1900.  September 
October 
November 
December 

1901.  January 
February 
March  . 
April     . 
May 

From  February  22  to  May  1  a  daily  advance  of  10  feet  was  made  without 
exception. 

No  difficulty  was  experienced  in  making  the  junction  with  the  Greenwich  shaft, 

243 


nil 

48  feet  0  inches 

50 

0 

101 

8 

168 

4 

195 

0 

260 

0 

218 

0 

141 

8 

TUNNEL    SHIELDS 

the  only  precaution  taken  being  the  driving  of  steel  needles  from  inside  the  shaft 
round  the  top  of  the  steel  plug,  so  as  to  form  a  hood  over  the  shield  when  it  reached 
the  shaft,  thereby  preventing  any  fall  of  the  ground  above  while  the  plug  was  being 
removed. 

The  pressure  in  the  Greenwich  shaft  was,  of  course,  when  the  shield  was  ap- 
proaching closely  to  it,  maintained  at  the  same  height  as  the  air  pressure  in  the 
tunnel. 


C.lron.  Segments 

H 


FIG.   157.     GREENWICH  TUNNEL,  LONDON. 
The  Shield  :    Longitudinal  Section. 


The  shield  was  designed  on  the  lines  of  the  modified  Mersey  Tunnel  shield  (see 
Figs.  146  and  147),  and  is  an  interesting  example  of  how  a  shield,  designed  on  lines 
which  had  previously  given  good  results,  was  modified  to  suit  improved  methods 
of  working  developed  as  the  tunnel  advanced. 

The  external  diameter  of  the  tunnel,  12  feet  9  inches,  rendered  any  horizontal 
division  of  the  shield  unnecessary,  as  the  difference  in  hydrostatic  pressure  between 
the  top  and  bottom  of  the  shield  was  comparatively  small . 

244 


THE    SHIELD    IN    WATER-BEARING    STRATA 

It  was  determined  therefore  to  use  a  shield,  as  said  above,  in  general  character 
resembling  the  Mersey  Tunnel  shield  as  altered  by  Sir  B.  Baker,  adding  to  the 
design,  however,  face  rams  of  a  peculiar  character  instead  of  the  provisional 
needles  which  had  been  then  employed,  and  certain  other  novelties  were  introduced. 

The  "  trap  "  or  "  box  "  arrangement  of  the  diaphragms  had  proved  so  useful 
at  the  Mersey  Tunnel 1  that  it  was  reproduced  at  Greenwich  with  but  small  modi- 
fications, and  the  other  details  of  the  shield  were  worked  out -in  subordination  to 
this  central  idea. 

The  shield  as  first  designed  is  shown  in  Figs.  157,  158,  159,  and  160. 

It  consists  of  an  outer  cylindrical  skin,  1  inch  in  thickness,  in  seven  strips, 
each  joint  having  a  1  inch  cover  over  it.  To  prevent  the  cover-plates  from  stripping 
when  the  shield  is  in  motion,  the  cast  steel  cutting  edge  is  thickened  in  front  of 
the  covers  (see  Fig.  158). 

The  cutting  edge  is  of  cast  steel,  formed  in  thirteen  segments,  with  close-fitting 
machined  joints,  each  segment  having  cast  on  it  two  teeth,  projecting  6  inches 
beyond  the  general  line  of  the  cutting  edge.  These  teeth,  which,  as  has  been  pre- 
viously mentioned  when  treating  of  the  Mersey  Tunnel  shield,  were  originally 
suggested  by  the  effective  action  of  similar  projections  on  the  grab  buckets  of  a 
dredging  crane,  proved  very  useful  when  working  in  ballast.  The  outside  of  the 
teeth,  except  when  these  came  in  front  of  one  of  the  cover  strips  of  the  skin,  pro- 
jected 1  inch  beyond  the  general  circumference  of  the  cutting  edge. 

In  coarse  ballast  this  projection  is  distinctly  advantageous,  but  it  is  not  suited 
to  the  system  described  by  Mr.  Haigh  in  the  Proc.  Inst.  C.E.,  vol.  cxxxix.,  p.  25,  by 
which  a  skin  or  coating  of  clay  originally  placed  ahead  of  the  cutting  edge  is  left  as 
the  shield  advances  on  the  outside  of  the  shield,  and  remains  to  form  a  more  or 
less  continuous  seal  to  retain  the  air  at  the  tail  of  the  skin. 

Immediately  behind  the  cutting  edge  is  placed  a  circular  box  girder  A,  I  foot 
8  inches  wide  by  1  foot  2  inches  deep,  formed  of  plates  and  angle  irons,  and  rivetted 
to  the  cylindrical  skin  of  the  shield,  while  it  is  bolted  to  the  cutting  edge  in  front 
and  the  cast-iron  segments  behind  it. 

A  circular  girder  of  this  kind  gives  great  stiffness  to  the  shield,  and  it  was  with 
this  idea  it  was  put  in  on  the  first  place.  Immediately  behind  it  is  a  continuous 
plate  diaphragm  B,  covering  nearly  one-half  the  sectional  area  of  the  shield,  and 
further  strengthened  by  the  horizontal  plates  forming  the  top  and  bottom  of  the 
face  rams'  boxes,  E,  E,  the  front  of  the  shield  being  thus  made  very  rigid.  The 
cutting  edge  and  skin  of  the  shield  projected  3  feet  beyond  the  vertical  diaphragm, 
and  the  circular  girder  stiffened  this  projection  infinitely  better  than  gussets  would 
have  done. 

In  fact,  the  combination  of  built  circular  girders  to  give  stiffness  with  an  actual 
cutting  of  cast  steel  in  front  is,  perhaps,  in  small  shields,  a  better  arrangement  than 
a  built  cutting  edge  of  plates  and  gussets  ;  the  latter  arrangement,  however  care- 
fully put  together,  being  always  liable  to  buckle  if  hard  material,  or  gravel  con- 
taining large  single  blocks,  is  met  with. 

In  case  of  crippling  it  is  much  easier  to  remove  and  renew  a  damaged  segment 
than  to  cut  out  and  make  good  a  crippled  plate-cutting  edge. 

In  this  shield  further  rigidity  is  given  to  the  front  by  a  vertical  stiffener  C, 
2  feet  8  inches  in  width  and  f  inch  thick,  bolted  to  the  cutting  edge  and  the  circular 
box  girder  at  top  and  bottom. 

1  See  page  228. 

245 


TUNNEL    SHIELDS 

The  diaphragm  B,  which  forms  the  front  portion  of  the  "  water  trap,"  the 
main  feature  of  the  shield,  is  pierced  with  numerous  holes,  some  covered  with  thick 
glass  for  observation,  others  smaller  for  inserting  bars  or  water  jets,  and  two  large 
rectangular  ones,  primarily  intended  for  giving  access  to  the  upper  face,  in  case 
of  necessity. 

No  use  was  made  of  them  ;  the  idea  of  washing  out  the  material  in  front  by 
means  of  water  jets  was  never  put  into  practice,  and  the  use  of  bars  for  loosening 
the  face  was  soon  found  to  be  impracticable. 


FIG.   158.     GREENWICH  TUNNEL,  LONDON. 
The  Shield  :    Front  Elevation. 

In  suggesting  the  use  of  water  jets  for  loosening  the  face,  the  designers  of  the 
shield  only  followed  earlier  inventors  who  had  patented  shields  for  use  with  water 
jets.  No  attempt,  however,  was  made  to  use  them  at  Greenwich. 

This  diaphragm  or  curtain,  f  inch  thick,  closed  entirely  the  upper  half  of  the 
shield  and  extended  downward  (below  the  boxes  containing  the  face  rams)  1  foot 
3  inches  below  the  horizontal  axis  of  the  shield. 

246 


THE    SHIELD    IN    WATER-BEARING    STRATA 

Below  this  level  were  removable  curtain  plates  D,  D,  of  f  inch  plate,  stiffened 
with  3  inch  by  3  inch  by  i  inch  angle  irons,  by  means  of  which,  when  all  were 
in  position,  the  front  aperture  of  the  shield  could  be  almost  completely  closed. 

These  lower  curtain  plates  were  never  used  in  actual  practice,  but  it  will  be 
seen  that,  by  providing  a  means  of  varying  the  depth  of  the  front  diaphragm,  it 
was  possible,  in  combination  with  a  similar  arrangement  in  the  back  diaphragm,  to 
vary,  to  a  very  large  extent,  the  amount  of  "  water-seal  "  or  trap  formed  by  the 


FIG.   159.     GREENWICH  TUNNEL,  LONDON. 
The  Shield  :    Back  Elevation. 


difference  of  level  between  the  bottom  of  the  front,  and  the  top  of  the  back,  dia- 
phragm. 

It  is  advisable  to  be  able  to  vary  the  amount  of  "  water-seal  "  or  difference 
of  level  of  the  plates  for  another  practical  reason.  All  circular  shields  after  travelling 
some  distance  revolve  on  their  axis,  and  it  may  well  happen  (as  indeed  happened 
at  Greenwich)  that  a  difference  in  level  between  the  bottom  of  the  front  diaphragm, 
and  the  top  of  the  back  one,  which  was  quite  sufficient  when  the  shield  started, 
may  practically  disappear  after  tunnelling  a  few  hundred  yards. 

The  boxes  E,  E,  containing  the  face  rams,  although  they  appear  to  destroy 

247 


TUNNEL    SHIELDS 

the  continuity  of  the  front  diaphragm,  really  form  part  of  it,  for  although  their 
front  end  is  open  to  allow  of  the  movement  of  the  face  rams,  the  rear  end  is  closed. 
The  plates  forming  the  top  and  bottom  of  these  boxes  were  f  inch  thick,  and  served 
as  horizontal  stiff eners  to  the  shield. 

At  each  end,  these  stiff eners  were  secured  to  castings  having  special  ribs  cast 
on  them,  and  forming  part  of  a  cast-iron  ring  G,  which  stiffened  the  shield  between 
the  front  diaphragm  plate,  and  the  diaphragm  to  which  was  attached  the  "  trap  " 
or  "  box  "  forming,  with  the  front  diaphragm,  the  water-seal. 

These  castings  were  bolted  together  with  hardwood  packings  at  the  joints. 

In  the  boxes  were  placed  the  face  rams,  which  were  of  a  peculiar  design. 


FIG.   160.     GKEENWICH  TUNNEL,  LONDON. 
The  Shield  :    Half  Sectional  Plan. 


The  cylinder  and  piston  were  of  the  conventional  type,  but  the  rams  had  the 
peculiarity  that  to  the  curved  cast-iron  heads  (F,  in  Fig.  158)  were  attached  curved 
tables,  the  edges  of  which  slid  on  angle  irons  fixed  to  the  framework  of  the  shield. 

The  idea  was  taken  from  the  way  in  which,  at  the  Mersey  Tunnel  (Vyrnwy 
Aqueduct),  iron  needles,  passed  through  holes  in  the  front  diaphragm,  had  held 
up  the  material  in  the  upper  part  of  the  face  while  the  miners  excavated  at  the 
bottom. 

But  in  practice  the  tables  were  found  useless,  and  were  soon  removed. 

The  ram  head  shaped  with  a  cutting  edge  also  was  found  of  no  assistance. 

The  most  serious  objection,  however,  to  the  face  rams  as  first  erected  was  that, 

248 


THE    SHIELD    IN    WATER-BEARING    STRATA 

when  extended  to  the  full  extent  of  the  cylinders,  the  front  of  the  ram  head  was 
only  in  a  line  with  the  cutting  edge  of  the  shield.  This,  for  working  in  loose  material, 
was  a  very  grave  defect  ;  face  rams,  except  for  working  in  London  Clay,  should  be 
capable  of  more  extension  forward. 

Immediately  to  the  rear  of  the  boxes  containing  the  front  rams,  and  secured 
between  the  cast-iron  segments  G,  G,  and  the  ram  castings  H,  H  (one  ram  only  is 
shown  in  Fig.  157),  were  plates  K,  K,  which  formed,  with  the  back  plate  of  the  face 
ram  boxes,  a  useful  stiffening  to  the  shield,  and  a  frame  to  which  the  "  trap  "  or 
"  box  "  of  the  shield  was  secured.  Above  the  face  ram  boxes,  the  plates  K,  K,  as 
shown  in  Figs.  157  and  160,  left  ample  working  space. 

The  trap  or  box  J  is  shown  clearly  in  Figs.  137  and  139,  and  was  made  of 
plates  |  inch  thick,  the  upper  portion  being  formed  of  removable  strips  similar 
to  those  already  described  as  being  at  the  bottom  of  the  front  diaphragm.  This 
box  was  also  provided  with  a  lid  in  two  sections,  but  owing  to  the  projection  behind 
of  the  two  face  ram  cylinders,  the  lids  or  doors  only  extended  over  about  one-half 
the  surface  of  the  box,  thus  considerably  hampering  the  work  of  clearing  out 
excavated  material  from  the  face  (see  Fig.  160). 

Around  and  above  this  box  were  fixed  inside  the  skin  of  the  shield  thirteen 
segments  H,  H,  forming  a  cast-iron  ring,  to  which  were  attached  the  main  rams  of 
the  shield. 

Beyond  them  the  skin  of  the  shield  extended  some  3  feet  6  inches,  the  stroke 
of  the  rams  being  about  1  foot  10  inches. 

These  cylinders  are  shown  in  Figs.  165  and  166  ;  they  were  7  inches  in  diameter, 
and,  with  hydraulic  compressors  working  at  1 J  tons  per  square  inch,  they  exert  a 
total  pressure  of  750  tons.  The  hydraulic  pressure  for  working  the  shield  rams 
was  supplied  by  two  compressors  placed  in  the  engine  room  in  the  contractors' 
yard  on  the  surface,  capable  of  compressing  up  to  3  tons  per  square  inch. 

The  shield,  constructed  as  described,  on  starting  from  the  shaft  sunk  for 
the  purpose,  commenced  tunnelling  through  open  water-bearing  ballast,  the  material 
for  dealing  with  which  the  arrangements  of  the  shield  were  especially  designed. 

The  general  scheme  of  working  was  to  excavate  the  lower  part  of  the  face 
under  the  shelter  of  the  face  rams  or  tables,  and  then  having  removed  a  length 
below,  to  withdraw  the  rams,  and  allow  the  material  above  them  to  fall  down,  assist- 
ing the  process,  if  necessary,  by  water  jets,  when  the  shield  could  advance  again  into 
the  face,  and  the  operation  be  repeated. 

After  some  trial  of  this  system,  it  was  abandoned,  it  being  found  that  the 
sliding  tables  of  the  front  rams  were  ineffective  for  the  purpose  for  which  they  were 
designed.  So  far  from  acting  as  temporary  supports  to  the  material  in  the  upper 
face  of  shield,  and  by  their  withdrawal  allowing  it  to  fall  and  leave  a  loose  area 
into  which  the  shield  could  advance,  it  was  found  that  the  actual  result  of  advancing 
the  tables  into  the  ballast  in  front  of  the  front  diaphragm  was  to  pack  the  ballast 
above  the  tables  and  against  the  front  diaphragm,  thus  practically  giving  the 
shield  a  solid  face  to  push  against. 

The  use  of  the  face  rams  was  discontinued,  and  in  their  place  "  needles," 
formed  of  small  rolled  joists,  were  tried,  openings  in  the  front  diaphragm,  immedi- 
ately above  the  face  ram  boxes,  being  made  for  the  purpose.  Some  success  at- 
tended this  method  of  working,  which  had,  at  an  earlier  date,  given  good  results 
in  the  Mersey  Tunnel ;  but  the  rate  of  progress  was  very  slow,  and  ultimately  the  use 
of  the  sliding  tables  on  the  face  rams  was  abandoned,  the  face  rams  themselves 

249 


TUNNEL    SHIELDS 

were  lengthened,  and  utilized  for  supporting  the  face  by  means  of  soldiers  sustaining 
ordinary  close  poling. 


«  ^2 


This  gave  satisfactory  results  from  the  first,  and  subsequently  the  method  of 
working  was  improved  by  excavating  in  front  of  the  cutting  edge,  and  filling  the 

250 


THE    SHIELD    IN    WATER-BEARING    STRATA 

circular  channel  so  made  with  clay,  thus  forming  an  airtight  medium  in  which 
the  cutting  edge  passed  when  the  shield  was  advancing. 

This  was  done  by  Brunei  in  the  Thames  Tunnel,1  in  that  case  to  keep  the 
water  out,  as  compressed  air  was  not  used  in  his  great  work  ;  and  in  recent  times 
on  the  Baker  Street  and  Waterloo  2  and  Waterloo  and  City  3  Railways,  with  the 
object  of  preventing  the  compressed  air  used  in  the  tunnel  from  escaping,  and 
facilitating  the  entry  of  the  cutting  edge  into  the  ballast.  The  use  of  the  clay 


FIG.   162.     GREENWICH  TUNNEL,  LONDON. 
Method  of  Working  the  Shield  in  Ballast.     Shield  at  end  of  Stroke. 


annular  space  round  the  cutting  edge  on  the  tunnels  of  the  two  Waterloo  Railways 
had  apparently  one  advantage  over  the  system  employed  at  Greenwich  Tunnel. 
The  clay  pockets  in  these  cases  were  excavated  a  little  wide  of  the  shield,  and  the 
cutting  edge  being  a  continuous  plate  with  a  bevelled  front  edge,  it  made,  as  the 

1  Weales'  Quarterly,  vol.  v.  (1846). 

2  See  page  269,  and  Proc.  Inst.  C.E.,  vol.  cl.,  p.  25. 

3  See  page  140,  and  Proc.  Inst.  C.E.,  vol.  cxxxix. 

251 


TUNNEL    SHIELDS 

shield  advanced,  a  clean  cut  into  the  clay,  and  so  leaving  outside  the  shield  a  skin 
of  that  material  which  remained   in   a   more   or   less  complete  state,  not  merely 


la 


o 


round  the  advancing  shield,  but  also  round  the  tunnel  behind   it. 
claimed  decreased  the  escape  of  air  at  the  tail  of  the  shield. 

252 


This    it   was 


THE    SHIELD    IN    WATER-BEARING    STRATA 

At  Greenwich,  the  clay  pockets  were  taken  out  the  bare  size  of  the  shield,  and 
on  this  account,  and  also  because  the  teeth  on  the  cutting  projecting  outside  the 
skin  of  the  shield  broke  up  the  clay  more  than  a  smooth  cutting  edge  would  have 
done,  no  continuous  clay  envelope  was  found  outside  the  tunnel  at  the  rear  of  the 
shield. 

Figs.  161  and  162  show  this  method  of  working,  which  differs  mainly  from 
previous  work  of  the  same  kind  in  the  use  made  of  the  face  rams. 

Beginning  at  the  top  of  the  shield,  the  ballast  was  removed  around  the  cutting 
edge  low  enough  to  fix  the  first  poling  boards,  which  in  the  first  instance  were 
held  in  position  by  short  timbers  stretched  from  the  frame  of  the  shield,  and  simi- 
larly all  the  face,  or  so  much  of  it  as  was  in  ballast,  was  poled,  each  poling  being 
temporarily  supported  in  the  same  way. 

The  poling  boards  did  not  extend  the  full  width  of  the  face,  but  were  in  two 
lengths,  thus  limiting,  when  the  ground  was  bad,  the  face  exposed  in  fixing  each 
poling  board  to  one-half  of  the  face  at  that  level.  Clay  was  placed  in  pockets 
opened  in  front  of  the  cutting  edge,  and  doubtless  lessened  considerably  the  work 
of  the  shield  rams,  but  the  provision  of  teeth  on  the  cutting  edge  was  found,  as  in 
the  Mersey  shield,  useful  in  this  respect.  When  all  the  face  was  poled,  the  face 
rams  of  the  shield  were  brought  into  action,  and  by  means  of  soldiers,  took  the 
pressure  of  the  face,  the  short  timbers  or  "clogs  "  which  had  previously  held  each 
poling  being  knocked  out.  When  once  the  pressure  of  the  face  was  taken  by  the 
face  rams,  the  shield  was  pushed  forward,  and  as  the  cutting  edge  passed  beyond 
the  polings,  the  face  rams  were  allowed  to  draw  back  at  the  same  rate  as  the  shield 
advanced,  so  that  while  the  latter  was  moving  forward,  and  so  allowing  space 
behind  for  the  erection  of  a  new  ring  of  the  tunnel  lining,  the  poled  face  remained 
unaltered  in  position.  The  position  of  the  shield  at  the  end  of  the  forward  move- 
ment just  described  is  shown  in  Fig.  162,  the  position  before  moving  being  shown 
in  Fig.  161. 

This  system  of  working  answered  admirably  when  working  in  open  ballast 
with  very  little  sand. 

The  material  met  with  for  a  considerable  length  of  the  tunnel  made  the  shield 
work  comparatively  easy,  and  the  "  trap  "  arrangement  was  consequently  prac- 
tically abandoned,  two  doors  being  cut  in  the  upper  front  diaphragm,  which  could 
be  closed  when  necessary  by  short  timbers  dropped  into  vertical  guides  on  either 
side  of  the  openings  (see  L,  L,  Figs.  161  and  163). 

Towards  the  conclusion  of  the  work,  however,  the  tunnel  was  again  made 
through  open  ballast  on  an  upward  gradient  of  1  in  15,  and  having  in  view  the  fact 
that  the  larger  portion  of  the  tunnel  already  built  was  lower  than  the  shield,  and 
that  consequently  any  blow  at  the  face,  unless  checked  by  the  shield,  must  result 
in  the  flooding  of  the  tunnel,  and  consequent  imprisonment  of  all  working  at  the 
face,  a  considerable  change  was  made  in  the  arrangements  of  the  shield. 

The  alterations  made  are  shown  in  Figs.  161,  163  and  164,  and  were  mainly 
in  the  direction  of  improving  the  means  of  exit  for  the  men  working  in  the  face 
in  the  event  of  an  inrush  of  water. 

As  before  mentioned,  doors  which  could  be  closed  by  short  poling  boards 
had  been  previously  cut  in  the  front  diaphragm. 

These  were  left  open,  the  polings  being  kept  in  readiness  if  required. 

A  second  closed  diaphragm  was  formed  at  the  rear  end  of  the  face  ram  box,  by 
fixing  to  the  plates  K,K  of  Figs.  157  and  159,  and  the  end  of  the  face  ram  boxes  a  frame 

253 


254 


THE    SHIELD    IN    WATER-BEARING    STRATA 


of  wood  and  iron  M,  M,  which  completely  closed  the  upper  part  of  the  shield,  except 
for  an  opening  about  3  feet  3  inches  by  2  feet.  This  opening  was  provided  with 
two  double  doors,  the  front  ones  open- 
ing towards  the  face,  and  the  back 
ones  to  the  rear  of  the  shield.  The 
front  pair  of  doors  were  held  closed  by 
a  short  stretcher  to  the  front  dia- 
phragm, which  could  be  easily  knocked 
away,  when  the  rush  of  air  outwards, 
which  a  "  blow  "  would  have  occa- 
sioned, would  make  the  doors  fly  open. 

The  doors  to  the  rear  of  the  new 
diaphragm  were  kept  open.  In  the 
event  of  any  "  blow  "  occurring  in  the 
face,  and  a  consequent  fall  of  the  bal- 
last blocking  the  ordinary  means  of 
egress  through  the  trap,  the  men  work- 
ing in  the  upper  part  of  the  face 
could  escape  through  these  doors,  and 
it  was  found,  on  the  few  occasions 
where  small  "  blows  "  did  occur,  that 
the  knowledge  of  a  means  of  escape 
thus  provided  did  embolden  the  men  to 
endeavour  to  make  things  safe  in  the 
face  before  leaving  instead  of  rushing 
out  of  the  shield  in  a  panic. 

With  this  arrangement  of  the 
shield  an  advance  of  5  feet,  or  three 
rings  of  the  cast-iron  lining  of  the 
tunnel,  was  made  daily  in  a  face  con- 
sisting almost  entirely  of  open  ballast. 

During  the  construction  of  the 
tunnel,  the  face  gave  way  on  several 
occasions,  and  the  trap  or  box  arrange- 
ment of  the  shield  proved  satisfactory 
each  time.  The  front  of  the  shield 
filled,  of  course,  with  gravel  and  water, 
but  at  no  time  did  any  great  quantity 
of  water  pass  over  the  trap  into  the 
tunnel. 

From  the  construction  of  the 
shield  it  will  be  seen  that  the  men 
working  at  the  face  were  shut  up  in  a 
very  small  chamber,  and,  in  conse- 
quence, in  spite  of  the  flow  of  air 
through  the  trap,  it  was  found  that 

the  atmosphere  in  the  lower  part  of  the  shield  was  always  more  charged  with 
carbonic  acid  than  that  in  the  open  tunnel  immediately  behind  the  shield.  On 
some  occasions,  when  the  amount  of  C02  at  the  invert  of  the  tunnel  immediately 

255 


""vfz-         -»<^L  v«z>-  Is*-/ 


' i 


TUNNEL    SHIELDS 

behind   the  shield  was  0-1  per  cent.,  there  was  0-2  per  cent,  at  the  invert  of  the 
front  of  the  shield. 


£  — • 

fc  5 

g  £ 

&  TT 


£-4 


A  simple  arrangement,  by  which  the  ordinary,  blow-out  pipe  of  the  tunnel 
was  utilized  to  remove  the  vitiated  air  in  the  shield,  proved  very  effective,  a  reduc- 

256 


THE    SHIELD    IN    WATER-BEARING    STRATA 

tion  of  30  per  cent,  in  the  amount  of  C02  in  the  shield  being  measured  by  analysis 
after  using  the  blow-out  pipe  for  a  few  minutes.1 

A  pipe  N  (see  Fig.  161)  of  3  inches  diameter  was  inserted  in  the  trap,  one 
end  of  it  reaching  to  the  bottom,  the  other  projecting  a  few  inches  through  the  vertical 
back,  of  the  trap.  On  this  projecting  end,  the  blow-out  pipe  of  the  tunnel,  when 
not  in  use  for  blowing  water  out  of  the  invert,  was  placed. 

By  this  arrangement  the  opening  of  the  valve  of  the  blow-out  pipe  drew  out 
the  air  from  the  bottom  of  the  shield  face,  and,  as  said  above,  a  noticeable  improve- 
ment of  the  air  in  the  working  chamber  was  effected  thereby.2 

The  shield  as  modified  proved  an  efficient  machine  for  working  in  water-bearing 
strata,  but  the  experience  gained  with  it  suggests  several  structural  improvements, 
which,  in  another  tunnel  of  approximately  the  same  diameter,  might  be  usefully 
carried  out. 

The  combination  of  the  built  ring  with  cast  steel  segments  forming  the  cutting 
edge  was  very  satisfactory,  but  the  length  of  the  front  chamber  so  made  might 
well  be  increased  from  3  feet  to  4  feet.  This  would  give  more  room  for  work,  and 
the  increase  in  the  length  of  the  shield,  an  important  matter  when  curves  have  to 
be  negotiated,  could  be  compensated  for  by  reductions  elsewhere. 

Such  an  increase  in  the  front  of  the  shield  should  be  made  in  the  invert  as 
well  as  in  the  crown. 

The  front  diaphragm  could  well  be  made  without  any  observation  holes,  but 
instead  be  constructed  in  hinged  segments  and  the  whole  bolted,  not  rivetted,  to  a 
circular  plate  introduced  behind  the  cutting  edge  circular  box  girder. 

The  distance  between  the  front  diaphragm  and  the  trap  opening  was  unneces- 
sarily great,  owing  to  the  sliding  tables  of  the  face  rams  having  to  be  withdrawn 
within  the  line  of  the  front  diaphragm.  The  front  rams,  if  unprovided  with 
sliding  tables,  need  only,  when  drawn  back,  be  within  the  shelter  of  the  cutting 
edge,  and  consequently  their  cylinders  could  be  correspondingly  advanced 
towards  the  cutting  edge,  and  their  extension  behind  the  front  diaphragm  re- 
duced, which  reduction  would  enable  the  "  trap  "  to  be  brought  nearer  to  the 
front  diaphragm. 

For  an  actual  application  of  this  idea,  see  the  drawing  of  the  Lea  Tunnel 
(Fig.  171). 

Among  the  smaller  improvements  made  in  the  shield  was  the  substitution  of 
Berry's  two-way  valves  for  the  ordinary  ones  usually  fitted  to  the  shield  rams. 

They  gave  good  results,  and  were  incomparably  better  as  regards  com- 
pactness. 

Details  of  the  shield  rams  and  of  the  face  rams  are  shown  in  Figs.  165,  166 
and  167. 

The  first  cost  of  a  shield  of  this  character  is  about  £1,700,  but  the  repairing 
charges  are  small,  and  when  the  Greenwich  tunnel  was  completed,  the  shield  was,  for 
practical  purposes,  as  good  as  ever. 

The  labour  necessary  to  work  the  shield  was  as  under,  the  wages  put  down 
to  each  man  being  those  on  which  the  piece-work  prices  were  based,  for  as  usual 
all  the  work  was  done  by  contracting  with  the  shield  gang,  a  progressive  bonus 
being  paid  on  the  number  of  rings  put  in  weekly  over  a  certain  number. 

1  Proc.  Inst.  C.E.,vol.  cl.,  pp.  20  and  22. 

2  For  compressed  air  experiments  in  this  tunnel,  see  p.  45. 

257  s 


TUNNEL    SHIELDS 


The  shield  gang  consisted  of- 


at   12s.  per  shift  of  8  hours 
10s. 
9s. 


1  ganger 

2  miners 

2  miners'  labourers 
5  labourers 
1  shield  driver  . 
1  locksman 

These  were  all  the  men  inside  the  airlock,  and  the  cost  of  their  labour,  including 
bonus,  varied  from  £10  per  foot  run  of  tunnel  when  only  one  ring  per  day  was 
erected  to  about  £4  5s.  when  three  were  daily  put  in. 

The  charges  for  air  compressors,  hoisting  apparatus,  lighting,  and  pumping 
vary  so  much  with  the  local  conditions  that  any  attempt  to  specify  them  would 
be  misleading. 

At  the  Greenwich  tunnel  the  first  cost  of  the  machinery  and  general  contractors' 
equipment  amounted  to  nearly  £20,000,  but  much  of  this  expenditure  was,  as  is 
usually  the  case,  spread  over  other  work  than  the  part  now  under  consideration. 

A  further  development  of  the  shield  described  above  is  illustrated  by  the 
machine  used  in  the  Lea  Tunnel. 

The  Lea  Tunnel  Shield  (1900) 

In  the  year  1900  it  was  decided  to  enlarge  the  outfall  sewers  of  the  London 
Main  Drainage  system  on  the  north  side  of  the  Thames,  and  this  necessitated 
tunnelling  under  the  River  Lea  at  a  point  near  Abbey  Mills  Pumping  Station,  where 
the  Lea  flows  in  three  channels.  It  was  decided  to  make  the  new  sewer,  11  feet 
6  inches  in  internal  diameter,  by  means  of  a  shield,  the  sewer  itself  to  be  of  cast  iron 
lined  with  masonry.  The  character  of  the  ground  at  the  sewer  level  was  reported 


FIG.   168.     THE  LEA  TUNNEL,  LONDON. 
Longitudinal  Section. 

to  be  open  water-bearing  ballast  above  and  clay  below  at  the  commencement, 
gradually  changing  to  a  full  face  of  clay.  In  actual  fact  the  material  described  in 
the  trial  holes  as  clay  proved  to  bs  for  the  greater  part  of  the  tunnel  merely  peat, 
but  when  the  shield  was  designed,  it  was  planned  with  the  object  of  making  it 
equally  useful  either  with  compressed  air  and  a  protected  face  or  with  ordinary 
atmospheric  pressure  and  an  open  face  of  clay  or  other  impermeable  material. 

The  tunnel  was  made  and  the  sewer  built,  but  partly  owing  to  insufficient 
air-compressing  power,  and  partly  owing  to  ignorant  handling  on  the  part  of  those 
entrusted  with  the  work,  the  shield  was  never  used  as  it  should  have  been,  and  as 
the  Greenwich  Tunnel  Shield  actually  was  being  used  at  the  time,  but  the  tunnelling 

258 


THE    SHIELD    IN    WATER-BEARING    STRATA 

was  carried  on  rather  on  the  lines  of  the  "  assisted  shield  "  method  referred  to  in 
a  previous  chapter. 

The  arrangements  made  for  building  a  tunnel  under  compressed  air  were  in 
general  of  the  same  character  as  in  other  works  of  the  like  nature,  but  some  modi- 
fications were  introduced,  due  to  the  special  circumstances  of  the  case. 

The  shield  chamber,  in  which  the  shield  was  erected,  was  made  in  brickwork, 
a  length  of  the  ordinary  brick  sewer  being  built  of  slightly  larger  diameter  to  permit 


FIG.   169.     LEA  TUNNEL,  LONDON. 
Vertical  Emergency  Airlock. 


of  the  erection  of  the  shield  within  it  (see  Fig.  168).  The  shield  was  erected  in 
the  open,  the  forward  end  of  the  chamber  where  the  iron  tunnel  was  to  commence 
being  close  timbered  during  the  process. 

When  the  shield  was  erected,  a  brickhead  with  airlock  A  (Figs.  15  and  16)  was 
built  behind  it,  and  work  under  compressed  air  proceeded  in  the  ordinary  way. 

259 


AM* 

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FIG.   170.     LEA  TUNNEL,  LONDON. 
Timber  Safety  Diaphragm  of  Screen  in  Tunnel. 


FIG.   171.     LEA  TUNNEL,  LONDON. 
The  Shield  :    Longitudinal  Section. 


260 


THE    SHIELD    IN    WATER-BEARING    STRATA 

It  was  necessary  to  provide,  at  the  point  where  the  shield  chamber  was  erected, 
a  ventilating  shaft  for  the  sewer  when  finished,  and  the  Author,  who  had  charge 
of  the  work  for  Sir  A.  R.  Binnie,  fitted  in  this  shaft  an  emergency  lock,  by  which  the 
miners  in  case  of  a  "  blow  "  in  the  face  which  blocked  the  main  airlock  could  escape. 
To  increase  the  conditions  of  safety,  a  safety  diaphragm  of  similar  character  to 
those  employed  at  Blackwall  and  at  Greenwich  was  employed.  In  a  small  tunnel 
like  the  one  under  consideration,  the  diaphragm  could  be  made  of  wood,  and  little 
labour  was  expended  on  it,  save  in  making  a  caulked  joint  with  the  iron  lining 


FIG.   172.     LEA  TUNNEL,  LONDON. 
The  Shield  :    Front  Elevation. 


of  the  tunnel.  The  boards  composing  the  diaphragm  were  tongued  and  grooved. 
Fig.  168  shows  the  arrangement  adopted,  by  which  it  is  evident  that,  could  the 
miners  after  a  "  blow  "  at  the  face  escape  behind  the  diaphragm,  they  would 
be  in  a  space  where  the  air  was  sealed  in,  and  where  they  would  at  any  rate  have 
temporary  protection  until  they  could  make  their  way  out  through  the  vertical 
airlock.  The  details  of  the  diaphragm  are  shown  in  Fig.  170. 

The  airlock  shown  in  detail  in  Fig.  169  was  of  the  simplest  character,  the 
two  ends  being  formed  of  i-inch  plates  built  into  the  brickwork  of  the  ventilating 
shaft,  in  which  plate  openings  for  the  lock  doors  were  cut.  When  the  compressed 

261 


TUNNEL    SHIELDS 

air  work  was  finished,  the  doors  and  fittings  were  removed,  and  the  end  plates 
left  in. 

In  general  character  the  shield  resembled  the  Greenwich  Tunnel  shield  and 
the  Baker  Street  and  Waterloo  subaqueous  shield  in  its  "  trap  "  construction, 
but  certain  modifications  were  introduced,  which  may  be  briefly  indicated. 

Figs.  171,  172,  173  and  174  show  how  nearly  the  general  arrangement  of  the 
shield  resembles  that  of  the  Greenwich  Tunnel,  with  the  important  difference  that 
the  whole  of  the  internal  framing  of  the  shield,  with  the  exception  of  the  vertical 


FIG.   173.     LEA  TUNNEL,  LONDON. 
The  Shield  :    half  back  elevation  and  half  section  through  Safety  Box. 

stiffener,  and  the  horizontal  plates  of  the  box  containing  the  face  rams,  could  be 
easily  removed,  and  replaced,  as  the  character  of  the  face  varied. 

The  characteristic  parts  of  the  "  trap  "  or  "  box  "  shield,  the  front  air-tight 
upper  diaphragm,  and  the  back  diaphragm  shaped  like  a  box  and  provided  with 
lids,  could  be  removed  with  ease,  leaving  a  shield  with  an  open  nearly  circular  face 
divided  by  vertical  and  horizontal  girders. 

This  is  shown  in  Fig.  173,  giving  a  half  back  elevation  and  half  sectional 
elevation  of  the  shield. 

262 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  face  rams  were  arranged  so  that,  when  extended,  they  were  well  in  advance 
of  the  cutting  edge  of  the  shield,  a  modification  made  as  a  result  of  the  working 


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of  the  Greenwich  shield,  and  by  thus  advancing  them  they  were  placed  nearly  clear 
of  the  doors  or  covers  of  the  "  trap,"  so  enabling  a  larger  width  to  be  opened  for 

263 


TUNNEL    SHIELDS 

working  purposes,  as  shown  in  Fig.  174.  (Compare  Fig.  160  of  the  Greenwich 
shield.)  An  improvement  was  also  made  in  the  attachment  to  the  frame  of  these 
face  rams.  They  were  secured  to  the  horizontal  plates  above  and  below  them  by 
bolts  passing  through  the  plates,  so  that  it  was  possible  to  remove  the  bolts,  and  so 
detach  the  rams,  without  having  to  timber  in  front  of  the  shield  (see  Fig.  175). 
On  the  other  hand,  the  circular-built  girder  of  the  Greenwich  shield  was  omitted, 
and  the  cast  steel  cutting  edge  bolted  directly  to  the  plates  to  which  the  removable 
front  diaphragm  was  attached.  It  was  found  that  the  reduction  in  the  distance 
from  the  front  of  the  cutting  edge  to  the  front  diaphragm  was  an  error  ;  the  working 
area  in  front  of  the  diaphragm  should  be  at  least  3  feet  as  in  the  Greenwich  Tunnel, 
and,  as  suggested  in  the  description  of  that  work,  could  well  be  made  4  feet. 

Quite  apart  from  the  greater  freedom  for  working  in  the  wider  chamber,  the 
miners  in  it  feel,  in  some  degree,  a  greater  sense  of  security  than  when  between 
a  working  face  and  a  diaphragm  only  1 8  inches  apart.  The  feeling  is  not  defensible 
on  any  logical  ground,  but  it  undoubtedly  exists,  and  the  men's  prejudices  must  be 
reckoned  with  in  making  the  arrangements  of  a  shield. 

The  front  diaphragm  of  the  shield  was  made,  as  regards  the  portion  above 
the  face  ram  boxes,  in  folding  sections,  but  this  arrangement  in  the  actual  circum- 
stances of  the  work,  was  never  tested.  In  fact,  as  said  above,  the  shield  was  never 
fairly  tested  ;  one  of  the  modifications  made  on  previous  designs  was,  however, 
distinctly  satisfactory,  namely,  the  making  the  face  rams  with  a  longer  stroke, 
or  rather  with  a  stroke  reaching  further  beyond  the  cutting  edge  of  the  shield. 

The  rams  used  with  this  shield  were  the  same  as  those  of  the  Greenwich  shield. 


The   Baker    Street   and   Waterloo    Railway    River   Shield.1 

At  the  same  time  that  the  two  tunnels  under  the  Thames  at  Greenwich  and 
the  River  Lea,  just  described,  were  in  progress,  two  tunnels,  forming  part  of  the 
Baker  Street  and  Waterloo  Railway,  were  in  course  of  construction  under  the 
Thames  at  Charing  Cross,  the  conditions  of  the  work  and  the  methods  employed 
being  in  many  respects  identical. 

The  head  of  water  to  be  dealt  with  was  approximately  the  same  ;  the  open 
ballast  through  which  all  the  tunnels  were  driven  was  of  the  same  character  ;  the 
shields  were  all  of  the  "  trap  "  or  "  box  "  type  ;  the  tunnels  were  nearly  identical 
in  size,  and  the  methods  of  timbering  the  face  were  on  much  the  same  lines. 

But  in  details,  there  were  in  the  Baker  Street  and  Waterloo  tunnels  sufficient 
variations  to  make  a  separate  description  necessary,  and  the  shield  in  particular  is 
interesting,  as,  like  the  one  at  Greenwich,  it  was  altered  in  its  frontal  construction 
after  the  commencement  of  actual  tunnelling  work,  the  original  design,  which 
was  modelled  on  one  previously  used  successfully,  failing  to  cope  with  the  conditions 
of  the  work. 

The  Baker  Street  and  Waterloo  Railway  Tunnels  are,  with  the  exception  of 
a  portion  of  the  length  under  the  River  Thames,  constructed  entirely  in  London 
Clay,  and  their  construction,  therefore,  for  the  greater  part  of  their  length,  is  iden- 
tical with  similar  work  described  in  chapter  IV. 

For  the  tunnels  under  the  river,  special  temporary  shafts  were  sunk  in  the 
river  itself,  from  which  the  shields  were  started,  and  which  were  removed  when 

1   Proc.  Inat.  E.G.,  vol.  cl.     Haigh  on  Subaqueous  Tunnelling  through  the  Thames  Gravel. 

264 


THE    SHIELD    IN    WATER-BEARING    STRATA 


the  work  was  finished.  This  arrangement  effected  a  notable  economy  in  the  work  ; 
all  the  material  for  the  construction  of,  or  excavated  from,  the  tunnels  was  water- 
borne  (see  Fig.  176). 

The  timber  stage  from 
which  all  the  tunnelling  opera- 
tions were  constructed  was 
370  feet  long  and  50  feet  wide, 
and  held  all  the  workshops, 
machinery,  stores,  etc.,  neces- 
sary for  the  work. 

The  boiler  power  provided 
amounted  to  300  nominal 
horse-power,  the  air  compres- 
sors being,  in  addition  to  a 
portable  Slee  engine  for  grout- 
ing purposes,  three  in  number, 
with  double  acting  air  cylinders, 
26  inches  in  diameter,  and  30 
inch  stroke,  and  running  up 
to  sixty  revolutions  per  minute. 

These  would  give,  assum- 
ing an  efficiency  of  80  per 
cent.,  about  140,000  cubic  feet 
of  free  air  per  hour. 

From  the  stage  cast-iron 
lined  shafts,  16  feet  in  dia- 
meter, were  sunk  (to  a  depth 
of  about  50  feet)  into  the  Lon- 
don Clay,  and  below  them 
again  brick  shield  chambers 
were  constructed  by  under- 
pinning. The  cast-iron  lining 
consisted  of  six  rings,  each 
8  feet  in  depth,  having  six 
segments  to  a  ring.  The  joints 
were  machined,  and  the  pieces 
were  put  together  with  red 
lead  and  hemp-yarn.  A  2- foot 
cutting  edge,  with  bevelled 
vertical  flanges  and  sharpened 
edge,  was  used  to  sink  the 
cylinder.  Six  rings  and  the 
cutting  edge  made  up  the  50- 
foot  length  of  iron  cylinder, 
reaching  about  10  feet  into 
the  London  Clay.  Below  this 

the  brick  chamber  extended  downwards  a  further  distance  of  22  feet.  To  prepare 
for  sinking  the  cylinder,  a  timber  platform  was  erected,  somewhat  above  the  level 
of  low  water,  and  on  it  the  cutting  edge  was  put  and  bolted  together.  The  two 

265 


TUNNEL    SHIELDS 

succeeding  rings  were  then  built  above,  and  the  length  was  slung  by  chains,  and 
held  by  union  screws  from  timber  balks  across  the  opening  in  the  stage-decking. 
The  platform  below  was  next  removed,  and  the  18  feet  of  cylinder  was  lowered 
through  the  mud,  the  grab  being  used  within,  until,  on  its  taking  a  bearing,  the 
screws  could  be  detached,  when  another  two  rings  were  built.  Including  the  weight 
of  the  cylinder,  a  load  of  106  tons,  equivalent  to  about  2|  cwt.  per  square  foot  of 


FIG.    177.     BAKER  STREET  AND  WATERLOO  RAILWAY,  LONDON. 
The  Shield  for  Subaqueous  Work  :    Longitudinal  Cross  Section. 


maximum  surface  exposed  to  friction,  sufficed  to  sink  the  remaining  6  feet  to  the 
required  level  for  underpinning  the  cylinder  with  the  brick  walls,  23  inches  thick, 
for  the  enlarged  chamber  below.  In  carrying  down  the  brickwork,  spaces  were 
left  for  eyes,  in  which  five-ringed  brindled  brickwork  in  2  to  1  Portland-cement 
mortar  was  afterwards  built,  forming  complete  circles  through  which  the  tunnelling- 
shields  were  to  be  driven,  the  work  above  being  propped- in  the  meantime.  Each  leg 
of  succeeding  rings  of  brickwork  was  built  under  the  junction  of  two  legs  of  its  pre- 
266 


THE   SHIELD    IX   WATER-BEARING   STRATA 


decessor.     Inverts  were  made  of  15-inch  Portland-cement  concrete,  dished  about 
6  inches. 

As  the  shafts  were  sunk  well  into  the  clay,  compressed  air  was  not  required 
in  starting  the  shields  from  the  shafts.  They  were  erected  in  the  brick  chambers, 
and  when  ready  to  start,  were  driven  into  the  clay  through  the  eyes  provided  for 
that  purpose,  and  a  short  length  of  tunnel  built  before^the  airlocks  were  put  in, 
and  compressed  air- work  started. 

These  tunnels  indeed  were  driven  under  the  Thames  for  the  greater  part  of 
the  distance  in  London  Clay,  but  for  a  part  of  their  length  they  passed  through 
a  bed   of   open   ballast    extending  up- 
wards   to   the    river,    containing    very 
little     sand,     and     being,     of    course, 
charged  with  water,  the  high  tide  level 
being  some  70  feet  above  the  tunnel 
invert  at  its  lowest  level. 

The  shield  employed  was,  there- 
fore, designed  to  work  under  these 
conditions. 

As  originally  constructed  it  is 
shown  in  Figs.  177  and  178. 

The  cutting  edge  A  was  formed 
of  four  plates,  |-  inch  thick,  and  cut  at 
the  face  to  a  bevelled  edge,  and  was 
shaped  into  a  "  hood  "  of  similar  pattern 
tD  that  devised  for  the  Waterloo  and 
City  Railway  shield.  The  plates  of  the 
cutting  edge  were  riveted  together 
with  1-inch  rivets  at  6  inches  pitch. 
The  segmental  plates  break  joint,  and 
over  the  six  joints  of  the  outermost 
plate  are  cover  plates,  which  are  ex- 
tended back  over  the  joints  of  the 
single  |-inch  plate  forming  the  tail  of 
the  shield.  Like  that  shield  too  it 
was  stiffened  with  a  circular  box 
girder  B,  to  the  rear  of  which  was 
attached  the  front  diaphragm  C,  and 
the  base  of  the  back  diaphragm  or  FIG.  1 78.  BAKER  STREET  AND  WATERLOO  RAILWAY, 
box  D.  LONDON. 

The  cutting  edge  projected  at  the  [  for  Su^5S  Wopk  : 

top  4  feet    3  inches    in    front    of    this 
front  diaphragm,  and  is  stiffened  by  gussets  E,  E,  attached  to  the  box  girder. 

The  shield  is  further  stiffened  by  a  vertical  central  framing  F  of  f-inch  plate, 
2  feet  9|  inches  wide,  similar  to  that  of  the  Greenwich  shield. 

The  box  or  trap  D  did  not  vary  in  essentials  from  those  used  elsewhere,  and 
proved  satisfactory,  for  though  no  serious  collapse  of  the  face  occurred  throughout  the 
whole  period  of  work,  the  occurrence  of  a  few  small  blows  from  time  to  time  demon- 
strated that  the  four  men  at  work  between  the  diaphragm  and  the  face  had  time 
to  escape  through  the  trap,  and  that,  on  filling,  the  trap  sealed  automatically, 

267 


TUNNEL    SHIELDS 

maintaining  an  unstable  equilibrium,  until  the  sand,  which  followed  from  the  face, 
filled  the  trap,  and  extended  with  a  sloping  surface  from  the  lip  of  the  diaphragm 
to  the  face  at  the  cutting-edge  soffit.  It  was  possible  to  remove  this  sand  sub- 
sequently, by  opening  carefully  under  the  edge  of  the  diaphragm,  until  a  miner 
could  gain  access  to  the  face,  and  set  protecting  timber.  The  top  of  the  trap  was 
provided  with  a  plate  wiHi  a  manhole  through  it,  and  a  covering  lid  which  could 
be  screwed  down  in  case  of  need.  Round  iron  foot-  and  hand-bars  were  fixed  up 
the  curved  sides  of  the  trap  and  inside  the  circular  girder. 

But  the  attempt  to  do  away  with  timber  work  in  the  face  by  fitting  the  shield 
with  sliding  shutters  was  an  innovation  in  a  shield  of  this  size. 

These  shutters  G,  G,  were  intended  to  cover  the  whole  working  face  of  the  shield 
to  within  3  feet  of  the  invert.  They  were  held  in  place  by  screws  H,  H,  H,  which 
worked  in  bearings  J,  J,  J,  fixed  to  the  skin  of  the  shield  and  to  the  vertical  frame 
F.  Each  shutter  was  in  two  independent  parts,  one  on  either  side  of  the  central 
vertical  stiffener,  so  that  in  bad  ground  one-half  of  the  face  could  be  worked  down 
at  a  time  and  so  minimise  the  risk  of  a  blow. 

It  was  intended  that  by  operating  these  screws  the  shield  could  be  driven 
forward,  while  at  the  same  time  the  face  of  the  ballast  could  be  sustained  by  the 
shutters,  which  would  be  made  to  give  way  as  the  shield  advanced.  The  Blackwall 
Tunnel  and  the  Hudson  River  tunnel  shields  had  been  fitted  with  similar  shutters, 
and  the  arrangement  had  proved  in  both  cases  perfectly  satisfactory.  These 
shields,  however,  were  of  much  larger  diameter,  and  in  the  case  of  the  Baker  Street 
and  Waterloo  shield,  it  was  soon  found  that  it  was  impossible  to  work  the  shutters 
at  the  top  of  the  shield  advantageously,  owing  to  the  limited  space  available,  and 
the  decision  to  abandon  their  use  was  no  doubt  also  influenced  by  the  fact  that,  for 
a  shield  of  this  size,  an  advance  by  means  of  shutters  successively  worked  meant 
a  very  slow  rate  of  progress,  as  compared  with  even  the  most  elaborate  system  of 
timbering. 

The  cast  steel  ring  composed  of  eight  segments,  which  carries  the  hydraulic 
rams,  was  of  the  usual  pattern,  but  the  disposition  of  the  rams  was  uncommon. 
They  were  fourteen  in  all,  and  of  this  number  eight  Avere  clustered  together  at  the 
lower  part  of  the  shield,  and  the  remaining  six  distributed  in  pairs  at  the  crown 
and  sides  (see  Fig.  179).  The  number  of  rams  is  in  excess  of  that  provided  pre- 
viously in  shields  of  the  same  size  ;  the  Mersey  (Vyrnwy)  Tunnel  shield  had  nine 
only,  and  the  Greenwich  shield  thirteen,  this  being  a  much  heavier  shield  than 
either  of  the  others. 

In  ordinary  conditions  of  work,  the  four  lowest  rams  were  not  brought  into 
use,  at  all,  and  of  the  remaining  ten,  those,  never  exceeding  six  in  number  at  once, 
were  employed  which  the  guiding  of  the  shield  required. 

The  rams  and  connexions  were  tested  for  a  working  pressure  of  2,400  pounds 
per  square  inch.  The  hydraulic  pressure  was  supplied  by  an  intensifier  fed  by 
water  from  an  hydraulic  power  company's  mains,  nominally  at  800  pounds  per 
square  inch.  In  this  intensifier  a  7-inch  cylinder  compressed  the  water  in  com- 
munication with  the  rams  by  two  3-inch  plungers  to  an  actual  pressure  of  2,400 
pounds  per  square  inch.  The  average  working  pressure  in  the  rams  was  1,300 
pounds  per  square  inch. 

The  tail  or  skin  K  of  the  shield  behind  the  circular-built  girder  consisted  of 
six  J-inch  plates  butt  jointed,  the  joints  being  covered  with  covers  8  inches  wide 
and  |  inch  thick. 

268 


THE    SHIELD    IN    WATER-BEARING    STRATA 

The  weight  of  the  shield  was  29i  tons. 

The  shield  had  hardly  got  to  work  in  bad  material  when  the  shutters  in  front 
were  found  to  be  inconvenient,  and  were  removed,  and  recourse  was  had  to 
timbering  ahead  of  the  shield,  somewhat  in  the  manner  already  described  in  the 
case  of  other  tunnels,  but  with  two  important  variations. 

The  use  of  clay  pockets  in  front  of  the  cutting  edge,  which  had  proved  so 
satisfactory  in  the  Waterloo  and  City  Railway,  was  adopted,  and,  by  the  employment 
of  steel  tubes  or  rakers  passing  through  holes  in  the  vertical  diaphragm  of  the 
shield,  the  use  of  the  timber  rakers,  such  as  were  used  on  the  City  and  South  London, 
and  Glasgow  District,  Railways  in  similar  conditions,  and  the  use  of  which  entirely 
destroyed  the  protective  character  of  the  shield,  was  done  away  with. 


FIG.   179.     BAKER  STKEET  AND  WATERLOO  RAILWAY,  LONDON. 
Shield  for  Subaqueous  Work  :    arrangement  of  Rams  and  Segments. 

The  timbering  of  the  face  varied  a  little  as  to  the  arrangement  of  the  polings, 
according  to  the  varying  nature  of  the  face.  Figs.  180  and  181  show  the  poling 
used  in  a  full  face  of  ballast,  and  also  the  steel  rakers,  and  their  relation  to  the 
shield,  which  appears  stripped  of  the  protective  shutters  in  front. 

Two  pairs  of  steel  struts  L,  L,  were  fitted,  constructed  of  steel  tubes,  5i  inches 
diameter  and  7  feet  6  inches  long.  These  tubes  were  closed  at  each  end  by  screw 
plugs,  that  at  the  forward  end  making  a  flush  end  with  the  tube,  so  as  to  give  a 
solid  bearing  against  the  walings  P,  P,  of  the  face,  that  at  the  rear  being  made  to 
receive  an  adjustable  head  M,  M,  by  which  the  strut  could  be  tightened  against  the 
transoms  or  byatts  N,  N.  These  struts  passed  through  the  diaphragms  of  the 
shield  in  holes  0,  0,  cut  for  the  purpose,  and  made  partially  air-tight  by  leather 
sleeves  fitted  on  the  pressure  side  of  the  diaphragms. 

269 


TUNNEL    SHIELDS 

The  byatts  were  secured  to  the  sides  of  the  tunnel,  and  held  tight  by  chogs 
between  the  flanges  of  the  tunnel  segments. 

The  face  polings  were  each  9  inches  by  3  inches  held  up  by  soldiers  R,  R,  9  inches 
by  6  inches,  the  whole  face  being  carefully  plastered  with  pugged  clay. 

The  use  of  pugged  clay  in  pockets  in  front  and  outside  of  the  cutting  edge  was 
efficacious  not  merely  in  making  an  air-tight  annular  space  into  which  the  shield 
could  easily  enter,  but  also  in  forming,  as  the  shield  wrent  forward,  an  air-seal  at 
the  joint  of  the  shield  and  the  last  tunnel  ring  erected. 

The  rate  of  working  was  usually  about  5  feet  per  day  of  twenty-four  hours, 
a  fair  advance  in  open  ballast. 


FIG.   180.     BAKER  STREET  AND  WATERLOO  RAILWAY. 
The  Shield  for  Subaqueous  Work  :    Method  of  Using. 


The  work  of  driving  the  west  or  down  tunnel  under  the  river  was  commenced 
on  March  19,  1900,  work  on  the  east  or  up  tunnel  being  postponed  until  the  first 
was  built. 

When  started  from  the  shaft,  the  shield  was  without  any  of  the  protective 
fittings,  as  there  was  over  the  tunnel  a  cover  of  clay  1 7  feet  thick,  and  the  river 
bed  was  7  feet  above  the  clay. 

Driving  under  ordinary  atmospheric  conditions  was  continued  until  April  2, 
when  the  advance  was  suspended  in  order  to  build  the^bulkhead  wall  with  airlocks 
behind  the  shield.  At  that  time  there  was  still  15  feet  of  clay  above  the  crown  of 

270 


THE    SHIELD^!  IN    WATER-BEARING    STRATA 

the  tunnel.1^  A  bulkhead,  8  feet  thick,  of  wire-cut  gault  bricks  and  Portland-cement 
mortar,  grouted  with  neat  cement  by  air  pressure  through  iron  pipes  built  in  for 
the  purpose,  formed  the  air-tight  diaphragm.  Besides  the  working-lock  5  feet 
9  inches  in  diameter  and  13  feet  6  inches  long,  there  was  an  emergency-lock  above 
it  3  feet  9  inches  in  diameter,  and  of  course  the  necessary  pipes  and  electric  wires 
were  built  in  the  brickwork  of  the  bulkhead. 

The  shield  was  re-started  on  May  2  after  one  month's  interval.  On  the  21st, 
when  the  clay  showed  a  change  in  quality,  the  shield  was  stopped,  and  the  fountain 
trap  at  the  back  was  attached.  The  cover  of  clay  at  the  cutting  edge  was  then 


FIG.   181.     BAKER  STREET  AND  WATERLOO  RAILWAY. 
The  Shield  for  Subaqueous  Work  :    the  Timbering  of  the  Face. 


5  feet,  and  the  depth  below  the  river-bed  18  feet.  Hitherto  a  box  heading,  with 
6-foot  timbers  for  head  and  side  trees  resting  on  sills,  had  been  worked  in  advance 
of  the  shield  to  a  distance  of  about  7  feet.  This  was  now  stopped.  Auger-borings 
were  being  made  in  the  length  each  time  the  shield  was  driven  forward,  extending 
to  5  feet  above  and  in  advance  of  the  cutting  edge  ;  they  were  also  made  in  advance 
of  the  box  heading.  On  May  23  the  lock-doors  were  closed,  and  an  air  pressure 
of  10  pounds  per  square  inch  was  maintained.  After  building-in  the  locks  and 
re-starting  the  shield,  one  engine  had  been  running  and  blowing  air  into  the  tunnel 
in  readiness  for  closing  the  door  at  any  time. 

On  beginning  compressed  air  work  the  labour-shifts  were  altered  from  two 

271 


TUNNEL    SHIELDS 

12-hour  to  three  8-hour  gangs.  On  June  6  the  cutting  edge  entered  the  gravel 
at  the  soffit.  For  about  three  days  previously  there  had  been  practically  no 
thickness  of  clay  cover  ;  and  the  vertical  face  at  its  upper  part  was  poled  up  and 
down,  middled  by  a  waling,  and  the  top  of  the  length  was  guarded  by  a  few  boards. 
When  ready  for  driving,  the  short  stretchers  against  the  shield  were  struck,  after 
substituting  a  waling  low  enough  to  be  held  by  the  hollow  steel  struts  which  passed 
through  the  diaphragm-plate,  and  were  blocked  against  a  byatt  9  inches  by  8| 
inches,  secured  in  the  flanges  of  the  last-built  ring  of  tunnel.  As  soon  as  the  thick- 
ness of  the  clay  cover  above  the  cutting  edge  had  diminished  to  only  2  or  3  inches, 
hand  holes  or  pockets  were  opened,  in  advance  of  the  face  and  in  front  of  the  cutting 
edge,  reaching  a  few  inches  above  it,  and  rather  more  than  one  length  in  advance, 
into  which  well-tempered  clay  was  put,  precisely  as  employed  on  the  Waterloo 
and  City  Railway  tunnels  ;  a  series  of  such  pockets  formed  finally  a  continuous 
portion  of  an  annular  bed  of  soft  clay,  into  which  the  cutting  edge  could  easily  enter. 
The  circumferential  length  of  this  annular  bed  grew  as  the  amount  of  ballast  face 
increased,  until  it  extended  round  the  whole  hood  ;  thus  forming,  as  the  shield 
advanced,  an  air-seal  at  its  tail  where  the  last  tunnel-ring  had  been  built,  and 
affording  a  clear  space  for  the  grout  to  enter  around  the  tunnel.  As  the  ballast 
face  grew  downward  a  second  row  of  poling-boards  was  introduced  below,  with  a 
middle  waling  9  inches  by  4  inches  ;  the  two  walings  were  held  by  soldiers  1 1  inches 
by  6  inches,  and  by  the  upper  pair  of  steel  struts,  when  the  shield  was  being  driven. 

On  June  16  the  special  18-inch  lining-rings  were  first  employed.  There  was 
then  15  inches  of  ballast  at  the  face,  and  a  cover  of  22  feet  to  the  bed  of  the  river. 
When  the  ballast  extended  down  3  feet  6  inches,  which  was  the  length  of  the  upper 
polings,  it  was  found  desirable  to  substitute  horizontal  planks  for  the  vertical 
upper  polings,  which  exposed  too  high  a  portion  of  the  face  on  removal ;  two  sets 
of  vertical  polings  continued  to  be  used  against  the  clay  below.  Each  of  the  three 
uppermost  widths  under  the  hood  consisted  of  two  pieces,  while  a  few  below  them 
were  long  planks  across  the  whole  face. 

From  June  21  to  27  the  top  pair  of  iron  shutters  was  used,  but  they  were  then 
abandoned  and  horizontal  timbers,  in  two  halves  across  the  face,  were  substituted. 
When  the  face-planking  came  into  use  as  far  down  as  the  lower  pair  of  steel  struts, 
the  planks  were  held  by  soldiers  and  walings  against  the  steel  struts,  the  arrangement 
of  the  timber  varying  to  suit  the  changes  in  level  of  the  ballast.  The  face  was  set 
forward  to  a  position  varying  between  2  feet  2  inches  and  2  feet  6  inches  beyond 
the  front  edge  of  the  vertical  girder.  The  whole  of  the  gravel  portion  of  the  face 
was  thickly  plastered  with  pugged  clay,  against  which  the  planks  were  set. 

On  July  15  there  was  a  full  face  of  ballast.  On  the  21st  a  run  of  water  filled 
the  fountain  trap.  On  August  20  and  21  blows  occurred,  filling  the  trap  with 
water,  followed  by  ballast  which  choked  itself  in  the  shield.  The  four  men  at  the 
face  escaped  safely  under  the  diaphragm-plate,  and  were  able  subsequently  to 
attack  the  face  again  by  getting  into  one  half-section  of  it  at  a  time,  the  vertical 
girder  forming  a  divisional  guard  exactly  adapted  to  the  circumstances.  On  both 
the  latter  occasions  it  was  the  uppermost  planks  which  were  blown  in.  It  is  probable 
that  on  August  20  more  timber  than  a  single  width  of  board  was  removed  simul- 
taneously for  setting  forward  ;  the  rule  was  one  width  only.  After  the  second  of 
these  two  blows  the  space  of  the  five  uppermost  planks  was  found  exposed,  the 
blow  occurring  when  the  lower  portion  of  the  face,  at  about  floor-level,  was  being 
set  forward. 

272 


THE    SHIELD    IN    WATER-BEARING    STRATA 

On  September  14  the  last  blow  occurred,  which  filled  the  fountain  trap  with 
ballast.  This  also  was  successfully  dealt  with,  but  involved  delay. 

On  September  21  a  good  deal  of  water  was  let  into  the  tunnel  from  deficiency 
of  air  pressure  ;  but  only  near  the  face  was  the  rail-road  awash.  From  this  time 
until  getting  completely  into  the  clay  the  ballast  was  coarse  and  open,  and  the  air 
escaped  so  readily  through  it  that  difficulty  was  experienced  in  keeping  up  the 
requisite  pressure.  The  blow-off  valve  of  the  air-receiver  was  regulated  auto- 
matically by  a  float  on  the  river,  which  carried  a  vertical  board  having  an  inclined 
channel-bar  groove  attached  to  it.  The  groove  constrained  a  roller,  which  moved 
horizontally  as  the  board  rose  and  fell  with  the  tide,  and  varied  by  its  movement 


r 


FIG.   182.     BAKER  STREET  AND  WATERLOO  RAILWAY. 
Dalrymple  Hay's  Hooded  Shield.     Details  of  Hydraulic  Ram. 


the  position  of  the  fulcrum  of  the  loaded  valve-lever,  thus  varying  the  pressure 
according  to  the  hydraulic  head. 

On  September  27  the  tunnel  re-entered  the  London  Clay  at  the  invert,  and 
on  October  6  a  full  face  of  clay  was  again  obtained.  During  straightforward  work, 
normal  progress  with  a  ballast  face  was  three  18-inch  rings  per  day  of  twenty-four 
hours. 

On  October  8  the  use  of  ordinary  20-inch  rings  was  resumed,  and  a  box  heading 
was  again  begun,  driven  as  low  as  possible.  On  the  24th,  at  low  tide,  all  the 
compressed  air  was  let  out  to  test  water-tightness.  There  were  droppers  through 
bolt-holes  and  some  joints  as  the  tide  rose.  Air-pressure  was  then  restored,  in 

273  T 


TUNNEL    SHIELDS 

order  to  fix  grummets  on  bolts  and  to  recaulk  some  joints.  On  October  27,  1900, 
the  air  pressure  was  finally  dispensed  with. 

The  construction  of  the  second  or  east  tunnel  closely  followed  the  procedure 
already  described  as  successful  in  the  west  tunnel,  and  the  work  was  completed 
without  incident. 

The  details  of  the  shield  ram  is  shown  in  Fig.  182. 

NOTE. — In  1885-6  a  Swedish  engineer  named  Lindmark  constructed  a  small  tunnel  in 
Stockholm,  in  which  he  employed  a  system  of  face  plates  supported  by  struts  from  removable 
iron  centres,  which,  though  it  can  hardly  be  considered  as  shieldwork  in  the  sense  in  which  the 
term  is  used  in  this  book,  was,  in  a  measure,  a  casing  or  protection  advanced  in  front  of  the 
tunnel  which  was  moved  forward  as  the  work  advanced,  and  may  be  briefly  described. 

The  tunnel  was  of  concrete  with  an  arched  roof,  straight  side  walls,  and  an  invert,  the 
inside  dimensions  being  :  height,  12  feet  3  inches  ;  and  width,  13  feet  6  inches.  The  ground 
passed  through  was  very  bad,  gravel  with  water  being  the  material  commonly  met  with.  The 
method  employed  for  supporting  the  face  was  to  support  it  by  struts  from  moveable  centres,  on 
which  the  concrete  tunnel  roof  rested.  The  face  itself  was  covered  by  small  plates  of  iron  about 
12  inches  square,  overlapping  at  their  edges  and  locking  into  each  other  by  T  bolts.  These 
plates  were  held  up  to  the  face  by  a  light  iron  centre  and  cross  framing.  As  each  plate  was 
detachable,  it  was  possible  to  remove  the  top  ones,  excavate  in  front  of  them,  and  reset  the 
plates  forward  of  the  rest  ;  and  by  working  downwards,  to  gradually  advance  the  whole  face. 

This  method  proved  fairly  satisfactory  for  some  time,  a  daily  advance  of  from  2  to  3  feet 
being  made,  but  after  a  time  the  amount  of  water  met  with  became  unmanageable,  and 
machinery  for  freezing  the  face  was  installed,  which,  in  conjunction  with  an  iron  face  plate, 
enabled  the  work  to  be  carried  to  a  successful  conclusion.  The  work  is  described  in  the 
Engineer  of  April  9,  1886. 


274 


Chapter    VIII 

THE  SHIELD  IN  MASONRY  TUNNELS 

THE  USE  OF  A  ROOF  SHIELD  IN  MASONRY  TUNNELS — THE  COLLECTEUR  DE  CLICHY  "  EX- 
TRA MUROS  " — THE  CHAGNAUD  SHIELD — DETAILED  DESCRIPTION — THE  CONVEYOR — 
METHOD  OF  WORKING  THE  SHIELD — THE  CENTRES  FOR  THE  MASONRY  ARCH — GENERAL 
WORKING  RESULTS — THE  COLLECTEUR  DE  CLICHY  "  INTRA  MUROS  " — DETAILS  OF  THE 
SHIELD — AND  OF  THE  CONVEYOR — THE  CENTRES  FOR  THE  MASONRY — THE  LAGGING — 
METHOD  OF  WORKING — GENERAL  WORKING  RESULTS — THE  SIPHON  DE  L'OISE — THE 
SHIELD  SIMILAR  TO  THE  EAST  RIVER  MACHINE — THE  AIRLOCK — THE  CONCRETE  LINING 
TO  THE  TUNNEL — DETAILS  OF  THE  IRON  CENTRES  AND  CASING — METHOD  OF  DRIVING  THE 
SHIELD  AND  COMPACTING  THE  CONCRETE  LINING — CONCRETE  LINING  COMPARED  WITH 
CAST  IRON— THE  PARIS  EXTENSION  OF  THE  ORLEANS  RAILWAY — DOUBLE  LINE  MASONRY 
TUNNEL — METHOD  OF  WORKING  WITH  ADVANCE  HEADINGS  FOR  THE  SIDEWALLS— DETAILS 
OF  THE  SHIELD — DESCRIPTION  OF  THE  WORKING — THE  CENTRES  FOR  THE  MASONRY — 
GENERAL  REMARKS 

THE  shields  described  in  the  preceding  chapters,  with  the  exception  of  Brunei's 
and  .the  experimental  Beach  Shield,1  have  worked  under  one  condition 
common  to  them  all,  namely,  that  the  excavation  carried  out  under  their  shelter 
was  immediately  and  permanently  protected  by  a  lining  of  cast  iron,  capable  of 
rapid  construction,  and  of  being  made  almost  entirely  water-tight,  without  the 
employment  of  skilled,  and  consequently  expensive,  labour. 

In  tunnels  constructed  in  water-bearing  strata,  under,  or  in  the  proximity  of, 
large  rivers,  cast  iron,  by  reason  of  its  ease  of  erection,  and  the  smaller  cross  sectional 
area  of  excavation  required  for  it  as  compared  with  brickwork,  will  probably  always 
be  a  more  satisfactory  lining  to  tunnels  than  brickwork ;  in  England  especially, 
where  the  price  of  cast  metal  is  low,  and  the  cost  of  tunnel  brickwork,  and  parti- 
cularly its  cost  in  labour,  high,  an  iron-lined  tunnel  compares,  in  cost,  not  too 
unfavourably  with  a  brick  one  of  similar  internal  area,  particularly  if  the  increased 
risk  of  settlement  in  the  latter  form  of  construction,  as  compared  with  the  former, 
be  taken  into  account. 

But  in  France,  where  the  cost  of  cast  iron  is  higher,  the  difference  in  cost  of  the 
two  methods  of  construction  is  greater,  and  after  the  first  employment  of  the 
Greathead  shield  in  the  Siphons  of  Clichy  and  of  the  Pont  de  la  Concorde,  in  both 
of  which  compressed  air  was  employed,  French  engineers  departed  from  the  model 
previously  adopted,  and  have  since  constructed  all  tunnels  built  under  a  shield  in 
masonry  instead  of  iron,  and  have  also  abandoned,  except  in  one  case,  the  circular 
form  of  the  shield  in  favour  of  the  type  now  known  as  the  "  roof  shield,"  or 
"  carapace."  It  is  true  that  all  these  later  works  have  been  built  under  conditions 
which  did  not  require  compressed  air.  Their  example  has  been  followed  in  the 
United  States,  and  the  tunnel  lately  driven  under  the  Harbour  at  Boston  is  lined 
with  concrete,  a  roof  shield  being  employed  in  its  construction,  in  conjunction  with 
compressed  air. 

1  See  page  14. 

275 


TUNNEL    SHIELDS 

The  roof  shield  is,  as  its  name  implies,  a  protection  for  the  upper  portion  of 
the  tunnel  only,  and,  although  its  form  and  the  method  of  erection  of  the  brick 
or  concrete  arch  behind  it  vary  considerably  in  different  tunnels,  the  manner  of 
its  use  may  conveniently  be  divided  into  two  classes  ;  namely,  the  mode  of  working, 
in  which  the  upper  portion  or  arch  of  the  tunnel  above  the  springings  is  first  built 
under  a  shield,  the  footings  and  invert  being  constructed  afterwards  by  under- 
pinning, and  secondly  the  driving  in  the  first  place  of  two  ordinary  timber  headings 
in  which  the  side  walls  of  the  tunnel  are  first  constructed,  and  then,  on  these  side- 
walls  as  bases,  a  shield  employed  to  excavate  the  roof. 

To  either  of  these  methods,  one  objection  is  obvious,  as  compared  with  the 
original  system  of  tunnelling  under  shield  as  set  forth  by  Greathead. 

The  great  merit  of  his  system  is  that,  in  a  period  to  be  measured  almost  by 
minutes,  the  excavation  for  a  tunnel  is  begun,  finished,  and  the  permanent  lining 
put  in,  the  chances  of  settlement  of  the  ground  above  being  thereby  reduced  to  a 
minimum. 

With  a  roof  shield,  the  excavation  is  attacked  in  three,  or  sometimes  in  four 
sections  (the  roof,  two  side  Avails  and  invert),  and  in  two  of  these,  the  excavation 
made  must  stand  on  timber  long  enough  to  build  in  the  masonry,  while  in  all  of 
them  green  masonry  must  take  the  earth  pressure  before  it  is  ready  for  it. 

These  objections  would  be  fatal  in  the  case  of  a  tunnel  in  water-bearing  ballast, 
but  in  the  case  of  large  tunnels  constructed  in  dry  material,  the  reduction  in  cost, 
due  to  the  use  of  brick  instead  of  cast-iron  lining,  and  the  increased  rate  of  travel 
of  a  roof  shield  of,  say,  30  feet  horizontal  width  over  a  circular  one  of  similar  internal 
road  capacity,  make  the  newer  method  very  attractive. 

Up  to  the  present,  no  tunnelling  has  been  carried  out  in  England  with  a  roof 
shield,  but  in  Paris,  and  in  the  United  States,  very  extensive  works  have  been, 
and  are  now  being,  executed  by  this  means. 

The  Collecteur  de  Clichy  (1895-9) * 

The  first  example  of  tunnelling  in  recent  years  with  a  shield,  the  tunnel 
lining  being  composed  of  masonry,  is  the  main  sewer  belonging  to  the  sewage 
system  of  Paris,  known  as  the  "  Collecteur  de  Clichy."  This  main  sewer  extends 
from  the  Place  de  la  Trinite  beneath  the  Rue  de  Clichy,  and  the  Avenue  de  Clichy 
(at  the  end  of  which  it  crosses  the  old  fortifications  of  Paris),  and  then  follows 
the  Boulevard  National  to  the  River  Seine  in  the  suburb  of  Clichy,  where  is 
situated  one  of  the  main  pumping  stations  of  the  city  drainage  system  (Fig.  222). 
The  engineer  in  charge  of  the  work  was  M.  Alphonse  Legouez,  who  is  well  known  as 
the  author  of  the  standard  French  work  in  this  class  of  tunnelling.  UEmploi  du 
Bouclier  dans  la  Construction  des  Souterrains,  Paris,  1897. 

The  construction  of  this  sewer  was  carried  out  in  two  sections,  the  one  com- 
prising the  length  outside  the  fortifications  of  Paris,  the  other  the  part  within  the 
city.  These  sections  corresponded,  as  it  happened,  almost  exactly  with  the  natural 
divisions  of  the  work,  that  outside  the  walls  of  Paris  ("  extra-muros  ")  being  for 
the  most  part  at  a  comparatively  small  depth  below  the  ground  level,  the  cover 
being  never  more  than  10  feet,  and  often  scarcely  2  feet,  over  the  crown  of  the 
sewer,  while  the  portion  within  the  walls  ("intra  muros  ")  was  for  the  greater 
part  of  its  length  many  feet  underground. 

1  Legouez,  Emploi  du  Bouclier,  p.  305,  and  Genie  Civile,  April  26,  1896. 

276 


THE    SHIELD    IN    MASONRY    TUNNELS 


Collecteur  de  Clichy  "  extra  muros  " 

The  portion  outside  the  walls  was  commenced  first  in  1895,  a  roof  shield  being 
employed,  and  later  (1896)  the  city  length  was  taken  in  hand,  with  a  shield  com- 
pletely enclosing  the  elliptical  barrel  of  the  sewer.  Throughout  its  entire  length 
the  sewer  was  lined  with  masonry. 

The  cross  section  of  the  sewer  is  shown  in  Fig.  183.  Its  internal  horizontal 
diameter  is  19  feet  8  inches,1  and  the  vertical  diameter  16  feet  5  inches  ;  in  the 
lower  half  of  the  ellipse,  however,  are  constructed  two  gangways,  each  about  3  feet 
wide,  leaving  a  waterway  of  13  feet.  The  thickness  of  the  masonry  varies  from 
16  inches  at  the  crown  to  2  feet  at  the  springing  line  and  18  inches  at  the  invert. 

An  uniform  gradient  of  1  in  2,000  was  maintained  throughout  (see  Fig.  184). 

The  portion  outside  the  walls    under    the  Boulevard  National,  a  crowded 
thoroughfare,  with  a  double  line  of  tramways  along  it,  was  offered  to  tender,  with 
the  condition  that  the  work  should  be 
carried    out  without    interference  with 
the  road  traffic  above. 

This  condition  could  hardly  be  ful- 
filled, if  the  ordinary  system  of  con- 
structing a  shallow  tunnel  by  cut  and 
cover  work  were  adopted,  and  the  old 
system  tunnelling  by  successive  tim- 
bered lengths  with  a  cover  of,  in  places, 
only  2  feet,  would  have  been  almost 
impossible  without  serious  interference 
with  the  road  traffic. 

The  shield  system  was,  therefore, 
adopted  by  the  contractor,  who  was 
successful  in  obtaining  the  work  of 
constructing  the  part  "  extra  muros  " 
about  5,750  feet  in  length  at  the  price 
of  1,016  francs  per  metre  lineal  or,  say, 

£38  14s.  per  lineal  yard.     As  in  each  yard  of  the  sewer  there  were  41  yards  of 
excavation  and  13|  yards  of  cement  concrete,  the  price  does  not  appear  excessive. 

For  this  the  contractor  undertook,  as  a  contingency  on  his  contract,  to  main- 
tain, at  all  times  during  the  construction  of  the  sewer,  the  street  traffic  unimpeded 
and  uninterrupted  above  over  the  full  width  of  the  road,  under  a  penalty  of  £20 
per  day  whenever  the  traffic  was  interfered  with. 

He  employed  a  shield  of  his  own  invention,  named  after  him  the  Chagnaud 
shield,  and  designed  with  two  principal  objects,  namely,  to  afford,  in  the  case  where 
the  cover  to  the  tunnel  was  very  slight,  a  broad  support  for  the  roadway  above, 
and,  in  the  second  place,  to  permit  of  the  erection  behind  the  shield  of  a  masonry 
tunnel.  The  broad  roof  he  obtained  by  advancing  the  cutting  edge  at  the  crown 
of  the  shield  considerably  in  advance  of  the  base,  and  so  forming  an  increased 
protection  for  the  workmen. 

The  masonry  tunnel  ring  was  made  possible  by  an  ingenious  modification  of 
previous  practice.  All  shields  used  to  that  date  pushed,  in  moving  forward,  against 
the  cast-iron  lining  already  erected,  but  this  could  obviously  not  be  done  in  the 

1  A  short  length  within  the  walls  was  some  3  feet  4  inches  less  in  horizontal  diameter. 

277 


FIG.   183.     MAIN  SEWER  AT  CLICHY,  PARIS. 
Cross  Section  of  the  finished  Sewer. 


TUNNEL    SHIELDS 

case  of  a  tunnel  lining  composed  of  masonry  concrete  or  brickwork,  which  requires 
a  considerable  period  to  set.  M.  Chagnaud  solved  the  difficulty  by  making  the 
rams  bear,  not  on  the  concrete  tunnel,  but  on  the  centres  supporting  it.  These 
centres,  some  thirty  in  number,  and  about  3  feet  3  inches  apart,  were  braced 
together,  and,  by  their  own  weight,  and  that  of  the  completed  tunnel  and  super- 
incumbent centre  above,  formed  a  sufficiently  solid  abutment  to  take  the  thrust 
of  the  shield  rams. 

In  the  shape  of  the  shield  also  M.  Chagnaud  was  the  first  to  depart  from  the 
circular  form  hitherto  employed. 


L 


FIG.   184.     MAIN  SEWER  AT  CLICK Y,  PARIS. 
Longitudinal  Section  (bottom). 

The  shield  had  its  roof  elliptical  to  suit  the  arch  of  the  sewer,  but  this  roof 
or  skin  was  cut  short  at  about  the  springing  line  of  the  arch,  and  the  bottom  of 
the  shield  was  formed  by  horizontal  girders  on  approximately  the  major  axis  of  the 
ellipse  of  the  roof. 

The  whole  structure  moved  forward  on  rollers,  which  in  turn  were  on  a 
bed  formed  of  timbers  extended  forward  in  short  lengths  as  the  shield  moved 
forward. 

The  shield  was  only  designed  to  enable  the  upper  half  of  the  sewer  to  be  con- 
structed under  it,  and  the  lower  portion  was  afterward  finished  in  sections  by 
underpinning  the  roof  arch  in  the  ordinary  manner. 

This  double  series  of  excavations  is  of  course  the  weak  point  of  the  roof  shield 
method  of  tunnelling,  and  it  was  perhaps  fortunate  for  the  inventor  of  the  new 
type  of  shield  that  the  material  passed  through  was  of  a  loose  sandy  nature,  making 
the  work  of  excavation  a  rapid  one,  and  thus  lessening  the  chances  of  settlement. 

278 


THE    SHIELD    IN    MASONRY    TUNNELS 

The  general  arrangement  of  the  shield  and  its  complementary  parts  is  shown 
in  Figs.  185  and  186.1 

It  was  practically  a  framed  roof,  the  envelope  or  skin  A,  to  use  the  usual 
English  phrase  for  the  covering  of  the  shield,  being  composed  of  plates  about  \  inch 
(0'55  inch)  thick,  the  joints  of  these  plates  being  at  right  angles  to  the  axis  of  the 
shield,  and  covered  inside  with  |-inch  plates.  The  skin  was  semi-elliptical  in 
shape,  the  major  axis  being  23  feet  9  inches  long,  and  the  semi-minor  axis  9  feet 
8  inches,  to  suit  the  extrados  of  the  arch  of  Fig.  183.  This  skin  projected  at  the 
crown  6  feet  11  inches  beyond  its  base  to  form  a  hood  i  to  shelter  the  miners  at 
work  at  the  face.  Originally  this  overhang  was  on^y  3  feet  11  inches,  but  later 
it  was  extended,  the  additional  gussets  L  fixed  to  support  the  extended  front  being 


FIG.    185.     MAIN  SEWEE  AT  CLICHY,  PARIS. 
The  Chagnaud  Shield  :  Longitudinal  Section  of  Shield  with  Conveyor. 


shown  in  Fig.  185,  where  the  altered  cutting  edge  i  is  also  indicated.     The  length 
of  the  shield  over  all  was  17  feet  3  inches. 

The  frame  consisted  essentially  of  two  curved  girders  J3,  B,  4  feet  7  inches 
apart,  which  were  braced  together  by  twelve  gussets,  and  at  their  ends  by  two 
horizontal  girders  C,  C,  which  extended  beyond  the  girders  B,  B,  to  the  tail  of  the 
shield,  to  which  they  were  connected  by  their  bottom  flanges,  these  latter  serving 
as  the  base  on  which  the  whole  framing  moved  over  the  cast-iron  rollers  K,  K  (see 
Fig.  186).  On  this  rear  portion  of  the  girders  C,  C,  rested  another  moveable  inde- 
pendent elliptical  girder  D,  supporting  an  arrangement  F  to  take  the  temporary 
poling  behind  the  shield.  The  girders  B,  B,  were  originally  prevented  from  spreading 

1  These  figures  are  reproduced  from  Genie  Civile,  by  courtesy  of  the  Editor. 

279 


TUNNEL    SHIELDS 
at  their  base  by  horizontal  girders,  which  were,  however,  removed  in  order  to 


increase  the  working  area,  even  after  the  shield  commenced  its  journey.     The 
remaining  ends  of  the  original  girders  are,  however,  shown  in  Fig.  186,  on  the  left 

280 


THE    SHIELD    IN    MASONRY    TUNNELS 


C.I.  shoe 


hand  in  full  lines  behind  the  electric  pump  P  and  its  motor  V,  and  on  the  right  in 
dotted  lines  behind  the  tank  W ' .  These  butt  ends  were  connected  by  two  channel 
irons  rivetted  back  to  back,  forming  the  girders  B'  B' ',  the  upper  flanges  of  the 
latter  being  bolted  to  the  lower  flanges  of  the  original  girders.  On  these  channels 
was  placed  the  working  platform  for  the  miners. 

The  cast-iron  rollers  K,  K,  under  the  frame  of  the  shield  were  7  inches  in 
diameter  and  1  foot  8  inches  long,  and  rolled  on  longitudinal  timbers  S,  S,  having 
on  their  upper  surfaces  plates  about  T40  of  an  inch  thick,  the  timbers  themselves  being 
made  in  short  lengths,  and  laid  in  advance  of  the  shield  on  the  floor  of  the  excavation. 

On  the  concave  flanges  of  the  girders  B,  B,  were  fixed  the  main  rams  of  the 
shield,  six  in  number,  9 J  inches  in  diameter,  and  having  a  stroke  of  3  feet  3  inches 
(1  metre).  These  rams  were  of  the  ordinary  double  action  type,  the  piston  head 
being  fitted  with  U-shaped  leathers,  but  the  pistons  instead  of  terminating,  as  in 
iron-lined  tunnels,  in  a  cast-iron  head,  which  bears  directly  on  the  last  tunnel  ring 
erected,  were  bolted  to  the  moveable  elliptical  girder  D,  the  ends  of  which  slid 
on  the  girders  C,  C,  forming  the  base  of  the  shield.  This  girder  or  arch  rib  D  was  of 
very  little  use  in  distributing  the  pressure  of  the  rams  evenly  over  the  masonry 
and  its  only  real  use  was  to  support  the  front 
ends  of  the  roof  polings  until  each  length  of 
these  was  caught  up  by  the  leading  centreing 
girder  G.  These  rams  were  driven  by  hydraulic 
pumps  P,  worked  by  a  motor  V,  and  supplied 
with  water  from  the  tank  N,  the  maximum 
thrust  per  ram  being  about  90  tons. 

The  ends  of  the  pistons  which  projected  a 
few  inches  beyond  the  moveable  girder  D,  bore 
on  the  last  iron  centre  G  erected,  which  in  turn 
was  braced  against  the  preceding  one  by  H-iron 
gussets  H,  placed  opposite  each  ram.  Usually 
some  thirty  of  these  iron  centres  were  erected 
at  one  time  all  braced  together  in  a  similar 

manner,  and  supporting  for  some  distance  behind  the  shield  polings  b,  Fig.  185, 
by  means  of  wedges  0,  O,  and  further  behind  laggings  laid  directly  on  the  centres, 
on  which  the  successive  lengths  of  tunnel  were  built.1 

A  neat  form  of  conveyor  worked  by  electricity  was  used  with  the  shield.  On 
a  bracket  at  the  crown  of  the  shield  a  small  motor  p  was  fixed  which  drove  by  a 
band  a  drum  p'  on  the  framed  girders  a,  b,  c.  This  drum  p'  had  on  its  shaft  a  mitre 
wheel  which  geared  with  another  on  the  shaft  r,  actuating  at  its  other  end  a 
shaft  carrying  a  drum  p" ' . 

The  conveying  band  m,  m,  was  stretched  over  this  drum  p"  and  another  at 
the  front  end  of  the  girder  a,  6,  c,  this  latter  being  adjustable.  The  whole  frame 
was  pivotted  on  a  block  fixed  on  the  rear  cross  girder  B'  of  the  shield,  the  rear 
end  being  sufficiently  elevated  to  discharge  the  material  carried  into  trucks. 

The  weight  of  the  shield  in  working  order  was  about  50  tons,  and  it  is  reported 
that  it  was  nearly  always  possible  to  move  this  weight  on  the  provisional  tram 
plates  s,  s,  without  serious  settlement  either  of  the  shield  or  of  the  thin  covering 
of  soil  above. 

1  These  moveable  centres  with  the  bracings  behind  them  resemble  those  of  Rhiza's  sys- 
tem (see  Drinker's  Tunnelling,  page  677). 

28l 


FIG.   187.     MAIN  SEWER  AT  CLICHY, 

PA  BIS. 

The  Chagnaud  Shield  :    Details  of    the 
Frame  and  Rollers. 


TUNNEL    SHIELDS 

The  working  pressure  necessary  for  driving  the  shield  in  ordinary  material 
of  little  consistency  was  about  200  tons,  in  harder  material  400  tons. 

The  mode  of  working  the  shield  is  as  follows.  When  the  shield  is  ready  to 
advance,  the  pistons  of  shield  rams  E,  E,  bear  on  the  centre  G  last  fixed,  the  moveable 
girder  D  moving  with  the  pistons,  thus  pushing  forward  the  shield  into  the  face 
which  the  miners  are  continuously  at  work  at,  and  rolling  it  over  the  rollers  K, 
which  are  constantly  being  freed  behind,  and  brought  forward  again  to  the  front. 

As  the  shield  moves  forward,  and  the  tail  of  the  roof  with  it,  thus  exposing 
the  polings  6,  the  wedges  o,  o,  on  the  penultimate  centre  are  tightened  so  as  to  make 
the  polings  close  the  space  left  by  the  withdrawal  of  the  tail.  If  this  is  carefully 
done,  no  settlement  above  occurs. 

The  ordinary  length  of  each  advance  was  equal  to  the  full  stroke  of  the  rams, 
namely,  3  feet  3  inches,  in  the  for  the  most  part  sandy  soil  traversed  under  the 
Boulevard  National,  beneath  which  the  portion  of  the  sewer  "  extra  muros  "  lies 
for  the  greater  portion  of  its  length,  and  the  time  occupied  was  usually  about  fifteen 
minutes. 

At  the  end  of  the  advance,  the  pressure  in  the  rams  was  reversed,  and  the 
moveable  girder  D  drawn  back  to  its  original  position  close  behind  the  rear  curved 
girder  B  of  the  shield  ;  a  new  centre  G  is  fixed  under  the  covers  of  the  shield,  new 
polings  b  are  fixed,  and  the  shield  is  ready  to  go  on. 

In  very  sandy  loose  soil,  a  very  thin  iron  plate  (one- half  of  a  millimetre) 
was  placed  outside  the  polings  and  under  the  shield  skin.  Its  use  prevented  the 
sand  coming  down  between  the  polings,  and  when  these  were  removed  one  by  one 
the  brick  lining  was  more  easily  put  in  if  the  plate  were  there.  At  first  some 
difficulty  was  experienced  by  the  skin  of  the  advancing  shield  drawing  the  thin 
plates  along  with  it  from  behind  the  polings,  but  this  was  got  over  by  bending 
the  plates  at  the  ends  over  the  ends  of  each  length  of  polings,  so  that  these  latter 
held  them. 

The  construction  of  the  permanent  masonry  lining  was  usually  in  progress 
some  yards  behind  the  shield.  The  polings  b  were  removed,  two  or  three  at  a 
time  with  the  wedges  supporting  them,  and  laid  directly  in  the  outside  flanges 
of  the  centreing  girders  G,  this  forming  the  lagging  for  the  arch,  which  was  built 
on  them,  and  then  the  next  two  or  three  polings  transferred,  and  so  on.  The  work 
was  so  arranged  that  successive  lengths  of  masonry  between  each  pair  of  centres 
were  in  different  stages  of  progress,  the  rearmost  length  where  work  was  going 
on  being  always  more  advanced  than  the  next  in  advance. 

The  only  cases  in  which  difficulty  was  experienced  in  building  the  brick  arch 
was  when  the  material  above  was  very  loose  sand,  but  even  in  this  material  it  was 
found  that  the  use  of  the  thin  plates  just  mentioned  stopped  in  a  large  measure 
the  formation  of  pockets  outside  the  tunnel  caused  by  the  falling  in  of  the  sand. 

It  was  not  the  least  advantage  of  this  ingenious  scheme  of  work  that  this 
removal  of  the  polings,  by  converting  them  into  laggings,  made  the  leaving  of  wood- 
work outside  the  brickwork  very  unlikely,  while  the  great  length  of  centreing 
necessarily  required  to  resist  the  thrust  of  the  shield  ensured  that  the  brickwork 
of  the  tunnel  would  be  supported  for  some  time  after  its  erection,  and  so  to  some 
extent  prevented  the  load  of  the  surrounding  material  being  thrown  on  the  brick 
when  actually  green. 

The  daily  actual  advance  of  the  shield  was  sometimes  29  feet  per  day  of  twenty- 
four  hours,  and  the  daily  average  over  a  period  of  221  days  was  14  feet  9  inches, 

282 


THE    SHIELD    IN    MASONRY    TUNNELS 

and  in  general  the  masonry  arch  behind  was  keyed  in  two  days  after  the  shield 
had  passed. 

The  remainder,  or  lower  half,  of  the  sewer  was  constructed  by  underpinning 
in  the  ordinary  manner. 

It  will  be  seen  that  an  important  factor  in  the  success  of  the  shield  was  the 
stability  of  the  two  platforms  or  roadways  on  which  the  shield  advanced.  These 
were  of  elm,  in  lengths  of  3  feet  3  inches  and  1  foot  8  inches  Avide,  covered  on  the 
upper  surface  by  an  iron  plate  T4(T  inch  thick  bolted  to  them.  They  generally 
rested  on  white  wood  planks  on  the  dry  sandy  material,  but  in  certain  spots  where 
water  was  met  with  in  the  sand,  short  piles  were  driven  to  support  them. 

The  work  of  introducing  these  roadways  in  front  of  the  shield  for  it  to  run  on 
was  one  which  required  care,  both  in  levelling  the  bed  to  receive  the  timbers,  and 
in  fixing  the  lengths  of  roadway  themselves. 

The  shield  suffered  but  little  in  the  construction  of  the  4,120  feet  of  tunnel 
built  under  it,  and  the  only  damage  done  to  the  structure  was  some  buckling  of 
the  cutting  edge  due  to  driving  into  some  old  masonry. 

M.  Legouez,  who  was  in  charge  of  the  work  states  1  that  thoroughout  the 
work  there  was  usually  a  settlement  of  the  ground  above  of  0*23  inches  (6  milli- 
metres), of  which  amount  0'115  inches  (3  millimetres)  was  caused  at  the  moment  of 
clearing  the  tail  of  the  shield,  and  a  similar  amount  during  the  construction  of 
the  masonry  lining.  This  latter  settlement,  he  says,  was  due  in  part  to  the  bad 
condition  of  the  centres  after  some  distance  had  been  driven.  They  become  deformed 
under  the  pressure  of  the  rams,  and  the  boltholes  in  their  joints  worn,  so  that  they 
were  hardly  in  a  state  to  maintain  a  sound  temporary  poled  roof  above. 

He  also  notes,  as  a  cause  of  movement  in  the  ground,  that  owing  to  the  use 
of  the  thin  metal  plates  behind  the  polings,  it  often  happened  that  cavities  formed 
during  the  advance  of  the  shield  were,  by  reason  of  being  behind  these  plates,  not 
discovered  in  time  to  prevent  settlement  from  them,  and  that,  by  the  use  of 
grouting  appliances,  all  movement  of  the  ground  behind  the  shield  could  have 
been  stopped. 

It  would  be  of  interest  to  know  to  what  extent  the  planks  on  which  the  tracks 
for  the  shield  were  laid,  and  which  remained  behind  to  serve  as  basis  for  the  centres, 
were  found  to  have  settled  by  the  time  the  brick  arch  was  finished  over  them. 

It  is  impossible  to  believe  that  some  part  at  least  of  the  settlement  of  the 
ground  was  not  due  to  the  sinking  of  the  shield  due  to  the  yielding  of  the  tracks 
beneath  its  weight,  and  it  is  at  least  probable  that  the  centres  when  first  fixed, 
however  carefully  the  work  was  done,  settled  a  little  when  the  weight  of  the  ground 
came  on  them,  on  the  advance  of  the  shield. 

But  however  that  may  be,  there  is  no  gainsaying  the  fact  that,  by  means  of 
M.  Chagnaud's  combination  of  shield  and  centreing,  this  large  tunnel  was  driven 
at  a  depth  below  the  surface  to  be  measured  in  some  places  almost  by  inches,  under 
a  busy  thoroughfare,  with  so  little  disturbance  of  the  ground  that  the  street  traffic 
above  was  never  interrupted. 

This  was  a  great  feat,  and  not  unnaturally  has  powerfully  influenced  the 
development  of  shield  tunnelling  in  Paris. 

The  construction  of  the  first  section  was  completed  in  1896,  and  the  second 
length,  "  intra  muros,"  or  that  portion  inside  the  old  fortifications  of  Paris,  was 
commenced  in  the  same  year. 

1  Emploi  du  Bouclier,  p.  325. 
283 


TUNNEL    SHIELDS 

Collecteur  de  Clichy  "  intra  muros  " 

This  second  section  was,  as  previously  stated,  driven  at  a  much  greater  depth 
than  the  first  section,  for  the  major  portion  of  its  length,  and  the  conditions  govern- 
ing its  construction  were,  therefore,  different. 

In  this  length  also,  the  contractor  who  undertook  the  work  employed  a  shield 
to  construct  a  masonry  tunnel.  While  reverting  in  one  important  feature  to  the 
English  type  of  shield,  by  making  it  cover  the  whole  area  of  the  sewer,  in  another 
he  went  beyond  the  innovation  introduced  by  M.  Chagnaud  in  discarding  the  use 
of  temporary  polings  behind  the  shield,  and  building  instead  the  permanent  masonry 
lining  within  the  shelter  of  the  shield  itself. 


10  '20  fL 

FIG.   188.     MAIN  SEWER  OF  CLICHY.  PARIS. 
The  Fougerolle  Shield  :    Longitudinal  Section  of  Shield  with  Conveyor. 

The  cross  section  of  the  sewer  was  the  same  as  that  of  the  portion  "  extra 
muros  "  (Fig.  183),  and  the  contractor,  M.  Fougerolle,  undertook  the  work  at  a 
price  of  about  £28  per  yard  of  length,  the  total  distance  being  about  2,730  yards. 

This  price  does  not  appear  excessive  in  view  of  the  nature  of  the  ground  to 
be  traversed,  which  for  half  the  distance  was  wet  sand,  and  for  the  remainder 
coarse  limestone  and  marl,  with  some  sand. 

The  shield  and moveable  centres  used  by  the  contractor,  M.  Fougerolle,  are  shown 
in  Figs.  188,  189,  and  190,  and  consisted  essentially  of  an  elliptical  skin  or  plating 
framed  on  two  built-up  girders  or  ribs  with  a  "  hooded  "  cutting  edge,  and  an  over- 
hanging tail. 

284 


THE    SHIELD    IN    MASONRY    TUNNELS 

The  two  main  ribs  A,  A,  were  6  feet  apart,  and  in  depth  varied  from  3  feet 
4  inches  on  the  vertical  to  4  feet  on  the  horizontal  axis  of  the  shield.  The  webs 
were  -j*0  inch  thick,  that  of  the  rearmost  one  being  perforated  in  twelve  places  to 
permit  of  the  shield  rams  projecting  through  it.  Between  these  main  ribs  were 
fixed  plate  gussets  B,  B,  arranged  in  pairs,  so  that  each  pair  with  the  webs  of  the 
main  girders  formed  a  box  in  which  a  shield  ram  C  was  fixed.  The  shield  was 
originally  designed  to  admit  of  twelve  rams  being  mounted  in  it,  but  only  eight 
were  actually  fitted,  three  at  the  crown  and  at  the  invert,  and  one  at  either  side. 

On  either  side  of  the  openings,  as  at  D,  where  provision  was  made  for  a  ram 
but  not  used,  there  were  fixed,  instead  of  the  plate  gussets,  wooden  frames  E,  E, 
which  served  equally  well  as  bracings  to  the  main  girders. 

The  skin  of  the  shield  consisted  of  plates  of  steel,  -^  inch  in  thickness, 
jointed  together  inside  by  covers  of  the  same  thickness,  the  joints  being  at  right 


FIG.   189.     MAIN  SEWER  AT  CLICHY,  PABIS. 
The  Fougerolle  Shield  :     Section  of  Tunnel  with  Centres  and  Rear  end  of  Conveyor. 


angles  to  the  axis  of  the  shield.     The  tail  of  the  shield  in  the  overhanging  portion 
consisted  of  two  thicknesses  of  T^  inch  steel. 

Round  the  main  girders  A,  A,  the  skin  formed  a  complete  ellipse,  and  this 
extended  3  feet  10  inches  in  front  of  the  leading  girder,  and  2  feet  2  inches  behind 
the  rearmost  one. 

The  projecting  hood  in  front  extended  8  feet  2£  inches  at  the  crown  in  front 
of  the  leading  girder,  and  the  overhanging  tail  which  came  down  nearly  to  the 
centre  of  the  shield  reached  9  feet  8  inches  behind  the  back  one.  Both  the  front  and 
the  tail  were  stiffened  with  gussets  F,  F. 

285 


TUNNEL  SHIELDS 

The  total  length  of  the  shield  at  the  crown  was  23  feet  10J  inches  (the  same 
as  the  horizontal  diameter)  and  at  the  invert  12  feet. 

On  the  front  gussets  in  the  crown  of  the  shields  was  fitted  a  channel  iron  G, 
just  sufficiently  away  from  the  skin  to  permit  of  polings  being  inserted  between 
so  as  to  support  the  roof  of  the  excavation  on  front  of  the  cutting  edge,  when 
necessary.  These  polings  were  pushed  forward  as  the  excavation  proceeded,  to 
hold  up  the  roof  of  the  excavation,  their  front  ends  being  driven  into  the  face. 
When  the  shield  advanced,  these  polings  being  free  to  slide  in  the  channel  G  slid 
back  under  the  hood,  whence  they  were  again  pushed  out  as  the  excavation  for 
the  next  length  advanced. 


FIG.   190.     MAIN  SEWER  AT  CLICHY,  PARIS. 
The  Fougerolle  Shield  :    Cross  Section  behind  rearmost  of  the  Elliptical  Girders  A,  A,  Fig.   188. 

The  upper  gussets  of  the  tail  were,  after  the  shield  had  started,  extended  so 
as  to  reach  within  18  inches  of  the  end  of  the  shield. 

Structurally,  the  most  obvious  defect  in  the  shield  is  the  absence  of  vertical 
stiffening.  The  shape  of  the  shield  suggests  that  deformation  is  likely  to  take 
place  by  flattening  the  crown,  this  being  indeed  usual  even  in  circular  shields, 
and  M.  Legouez  states  x  as  a  fact  that  it  was  found  necessary  to  strut  the  ribs  A,  A, 
by  vertical  timbers  on  either  side  of  the  conveyor  which  occupied  the  centre  of 
the  shield. 

The  amount  of  overhang,  the  crown  of  the  skin  being  double  the  length  of  the 

1  Emploi  du  Bouclier,  pp.  347-8.  The  Author  is  indebted  to  M.  Legouez  for  the  drawings 
from  which  Figs.  188,  189,  and  190  are  prepared. 

286 


THE    SHIELD    IN    MASONRY    TUNNELS 

invert,  appears  very  great,  though  not  so  excessive  as  in  some  later  shields  employed 
on  the  Metropolitan  Railway  of  Paris,  and  as  a  result  of  the  absence  of  any  stiffening 
round  the  cutting  edge,  some  buckling  occurred.  ^ 

The  rams  were  originally  made  of  cast  iron,  but  cast  steel  cylinders  were 
subsequently  substituted.  They  were  worked  by  electrically  driven  pumps  H 
fixed  on  the  upper  platform  J  of  the  shield  between  the  main  ribs,  the  usual  pressure 
employed  being  about  180  tons  on  the  eight  rams,  or  2|  tons  per  ram.  The  rams. 
9|  inches  in  diameter,  had  a  stroke  of  about  2  feet,  and  as  the  cylinders  were  single- 
acting,  they  were  drawn  back  at  the  end  of  the  stroke  by  a  pinion  K,  worked  by 
hand  and  gearing  into  a  rack  fixed  in  the  piston,  which  was  of  exceptional  length, 
and  supported  in  a  cast  steel  guide. 

The  working  platforms  in  the  shield,  in  addition  to  the  platform  /  just  men- 
tioned, were  in  front  two  in  number,  carried  on  channel  irons  L,  L,  and  on  them 
the  miners  worked  at  the  face.  Behind,  in  the  tail  of  the  shield,  was  another  plat- 
form M,  on  which  the  masons  worked  when  building  the  arch  of  the  tunnel. 

The  material  excavated  was  conveyed  to  the  skips  behind 
the  shield  by  means  of  a  mechanical  conveyor  about  80  feet 
long,  the  front  end  of  which  was  supported  on  and  fixed  to  the 
rearmost  of  the  two  main  ribs  of  the  shield  at  N,  the  other  end 
being  carried  on  a  truck  P.  The  machine  therefore  moved 
forward  with  the  shield. 

It  consisted  of  two  girders  Q,  Q,  of  \  inch  plates,  2  feet  6 
inches  apart,  stiffened  with  angle  irons  on  the  upper  edge,  and 
channel  bars   on  the   lower.     These  latter  served  also  as  the 
lower  guide  for  the  travelling  carrier,  the  upper  ones  being 
angle  irons.     At  either  end  of  these  two  girders  were  fixed  be-    FIG.  191.  MAIN  SEWER 
tween  them  drums,  the  front  one  R  being  on  an  axle  fitted  in        AT  CLICHY,  PARIS. 
slots  on  the  girders,  and  adjustable  by  means  of  screws  8,  by    T11?  Fougeroiie  Shield: 

.  .  ,  '  .J  ,     J  ;,,  J         Detail  of  Wedges  in 

which  arrangement  the  carrier  was  kept  taut,  and  the  rear  one  Centres  behind  Shield. 
T  being  geared  with  a  chain  of  wheels  driven  by  a  band  from 

an  electric  motor  carried  on  the  truck  P.  The  carrier  itself  V  consisted  of  an 
endless  chain  of  buckets  or  dishes  2  feet  2  inches  wide,  connected  together  by  pins, 
the  ends  of  which  slid  in  the  angle  guides  fixed  to  the  girders.  The  rear  end  of 
the  machine  was  placed  sufficiently  high  to  enable  the  skips  to  be  brought  be- 
neath it  as  shown  in  Fig.  189.  The  motor  driving  the  carrier  was  of  12  horse- 
power, and  the  daily  consumption  of  current  100,000  watts. 

The  shield  was  driven  forward  against  the  centres  a,  a,  about  2  feet  apart, 
which  were  connected  together  by  distance  pieces  b,  b,  of  cast  iron,  these  latter  being 
placed  opposite  the  shield  rams,  so  as  to  receive  their  thrust  and  transmit  the 
pressure  from  the  leading  centre  and  distribute  it  among  the  thirty  or  more  centres 
which  were  usually  in  position  at  one  time.  The  cast-iron  distance  pieces  were 
an  improvement  on  the  H  irons  used  in  a  similar  capacity  in  the  shield  of  the 
"  extra  muros  "  section  of  the  sewer,  the  broad  ends  of  the  castings  making  a  better 
bearing  than  the  sawn  ends  of  the  H  irons  against  the  centres. 

The  centres  themselves  were  made  of  plates  about  14|  inches  deep  and  0'23 
inches  thick,  stiffened  with  two  angles  at  top  and  bottom,  2|  inches  by  2i  inches 
by  0*23  inches.  For  purposes  of  adjustment  they  were  made  in  two  pieces,  the 
ends  being  bolted  together  and  timber  wedges  driven  between  them  as  shown  at 
c,  c,  Figs.  188  and  189,  and  in  detail  in  Fig.  191. 

287 


TUNNEL    SHIELDS 


ror\    2" 


Each  of  these  pieces  was  made  in  two  sections  for  convenience  of  moving  about, 
and  secured  together  when  in  position  by  fish  plates.  As  in  the  Chagnaud  contract, 
these  centres  proved  somewhat  weak,  and  even  though  the  labour  of  handling 
them  would  have  been  greater  and  the  obstructions  in  the  tunnel  greater  had  they 
been  made  deeper  and  stouter,  the  results  would  probably  have  been  more  satis- 
factory. 

As  an  additional  security  against  movement  of  the  centres  under  the  pressure 
of  the  shield  rams,  anchor  plates  d,  d,  were  fixed  in  the  masonry  lining  of  the  tunnel, 
which  by  means  of  bolts  clutched  the  distance  pieces  between  the  centres.  These 
anchor  plates  were  fixed  about  8  feet  apart,  being  built  into  the  brickwork  when 
the  latter  was  being  built,  but  they  were  not  used  until  the  brickwork  had  had 
time  to  set. 

The  lagging  e,  e  (Fig.  188)  used  on  the  centres  was  made  in  strips  ot  inch  wood 
2  feet  wide  (to  fit  the  centres  and  about  4  feet  7  inches  long,  covered  Avith  thin 
metal  plates  and  stiffened  lengthwise  by  two  channel  bars  2  inches  by  1  inch  by 
•23  inch.  These  were  convenient  for  handling,  and  the  channel  bars  kept  the 
wide  boards  from  warping  (Fig.  192). 

The  working  of  the  shield  was  on  the  ordinary  lines,  the  ordinary  length  of 
each  push  being  2  feet,  the  distance  apart  of  the  centres  for  the  masonry.  The 
excavation  was  taken  out  in  steps,  as  shown  in 
Fig.  188,  and  in  advancing  the  shield  the  only 
precaution  necessitated  by  the  fact  that  the 
length  of  masonry  over  each  new  centre  was  put 
in  under  the  shelter  of  the  shield  itself  was  the 
keeping  free,  as  far  as  possible,  the  tail  of  the 
shield  from  the  masonry,  so  that  the  latter  was 
not  drawn  forward  with  it. 

This  was  done  by  driving  wedges  under  the 
tail  and  so  reducing  the  friction. 

The  construction  of  the  masonry  lining  was 
usually  carried  out  in  steps,  the  work  from  invert 
to  crown  being  spread  over  about  6  feet.  The 
invert  was  put  in  immediately  behind  the  shield  on  the  area  left  bare  as  the 
shield  advanced  ;  the  side  walls  were  carried  up  by  another  gang  between  the  lead- 
ing centre  and  the  next  one,  while  a  third  set  of  men  turned  the  arch  under  the 
overhanging  tail  of  the  shield. 

This  arrangement  enabled  the  lower  half  of  each  successive  centre  to  be  fixed 
in  advance  of  the  upper  portion,  and  consequently  the  lower  ranis  of  the  shield 
were  made  with  shorter  pistons  than  those  in  the  upper  part. 

The  masonry  of  the  arch  consisted  of  concrete  blocks,  and  cement  mortar 
mixed  in  the  proportion  of  350  kg.  of  cement  to  1  cubic  metre  of  sand,  or  say  1  to 
5  by  weight  nearly. 

The  average  rate  of  progress  was  about  10  feet  per  day  of  twenty-four  hours, 
and,  compared  with  the  rate  of  progress  of  the  roof  shield  employed  in  the  "  extra 
muros  "  section  ot  the  tunnel  when  the  upper  half  only  of  the  work  was  constructed 
at  the  rate  of  about  14  feet  per  day,  must  be  regarded  as  very  satisfactory. 

In  the  working  of  the  shield  two  occurrences  are  of  interest. 

As  at  Black  wall,  the  plates  forming  the  skin  became  buckled  under  the  pressure 
of  the  ground,  but  in  the  case  of  the  Clichy  shield  it  was  the  tail  which  gave  the 

288 


I"  plonk 


FIG.  192. 


MAIN  SEWER  AT  CLICHY, 

PARIS. 
Detail  of  Lagging  for  Centres. 


THE  SHIELD   IN   MASONRY   TUNNELS 


greatest  trouble,  and  from  the  fact  that  the  thickness  of  the  masonry  arch  was 
determined  by  it,  it  was  important  to  keep  this  in  shape. 

The  tail  collapsed  under  the  pressure  of  the  wet  sand  over  it,  and  that  being 
precisely  the  spot  where  the  full  thickness  of  masonry  was  necessary,  the  shield 
was  stopped  and  the  defective  plates  were  removed.  This  was  done  by  opening 
up  from  the  front  of  the  cutting  edge  a  timbered  heading  along  the  top  of  the  shield, 
from  which,  when  the  tail  end  was  reached,  a  transverse  heading  was  driven,  in 
which  the  roof  plates  of  the  shield  were  removed  and  after  recurving,  replaced. 
The  headings  were  subsequently  filled  with  concrete. 

The  other  novel  feature  in  the  working  remains,  so  far  as  the  Author  is  aware, 
unique.  As  stated  above  (page  277),  a  portion  of  the  sewer  within  the  fortifications 
is  of  smaller  section  than  the  remainder,  and  on  reaching  the  point  where  the 
reduced  section  ended,  the  shield  itself  was  reduced  in  size  to  deal  with  the  smaller 
sewer.  This  was  done  by  excavating  around  one-half  of  the  shield  (see  Fig.  193), 
leaving  the  other  half  embedded 
in  the  ground,  and  against  the 
timbered  sides  of  the  chamber 
so  made  the  exposed  portion  of 
the  shield  was  strutted,  and  piece 
by  piece  the  members  and  skin  of 
the  shield  were  detached,  their 
superfluous  parts  removed,  and 
put  together  again,  with  the  re- 
sult that,  from  having  a  cross 
section  of  elliptical  form  with  a 
major  horizontal  axis  of  23  feet 
10 1  inches  and  a  minor  vertical 
axis  of  19  feet  5  inches,  it  be- 
came an  ellipse  20  feet  in  hori- 
zontal and  19  feet  in  vertical 
diameter,  the  decrease  in  vertical 
height  being  due  to  sagging  of 
the  shield. 

The  result  of  this  courageous 
experiment  was,  on  the  whole,  satisfactory,  though  considerable  trouble  was  sub- 
sequently experienced,  due  to  the  fact  that,  from  the  conditions  in  which  the 
change  of  shape  of  the  shield  was  made,  the  new  ri vetted  joints  of  the  shield 
girders  were  hardly  sufficiently  well  executed  to  resist  the  pressure  they  had  to 
support.  When  these  joints  yielded,  the  shield  spread  and  it  was  necessary  to 
repair  again  the  girders.  In  spite  of  this,  however,  the  shield  finished  its  course 
satisfactorily. 

The  settlement  of  the  ground  above,  due  to  the  works  of  the  sewer,  appears 
to  have  been  insignificant  in  amount,  and  from  this  point  of  view  the  undertaking 
was  a  complete  success. 

The  centres,  however,  again,  as  in  the  section  "  extra  muros,"  proved  too 
weak  for  the  work  they  had  to  do,  and  the  anchor  plates  gave  them  but  little 
support. 

The  absence  of  the  moveable  girder  used  in  M.  Chagnaud's  shield  was  doubtless 
an  advantage,  in  enabling  the  shield  rams  to  act  more  independently  when  it  was 

289  u 


Fio.   193.     MAIN  SEWER  AT  CLICHY,  PARIS. 
Reduction  in  size  of  Fougerolle  Shield. 


TUNNEL    SHIELDS 

required  to  deflect  the  shield  up  or  down  or  sideways,  but  the  number  of  rams 
was  small  for  a  shield  of  the  dimensions  described,  and  any  unequal  pressure  on 
the  centres  due  to  the  thrust  of  the  shield  being  applied  partially  by  some  of  the 
rams  only  had  the  effect  of  distorting  them,  and  so  necessitating  subsequent 
refitting  and  increased  cost  in  re-erection. 

An  increase  in  the  number  of  rams,  and  the  fitting  them  with  T-shaped  shoes, 
while  it  would  have  necessitated  an  increased  number  of  struts  between  the  centres, 
would  have  avoided  much  of  the  expense  actually  incurred  in  maintaining  in  good 
order  the  centres,  and  have  minimised  the  trouble  occasioned  by  the  breaking  down 
of  single  rams  from  time  to  time. 


The  Siphon  de  1'Oise  (1897) 

The  success  which  had  attended  the  employment  of  the  shield  in  conjunction 
with  masonry  in  the  work  just  described  led  to  the  use  of  similar  machines  in  other 
sewer  works  in  and  around  Paris.  For  the  most  part  the  details  of  the  operations 
varied  but  little  from  the  methods  employed  in  the  second  section  of  the  Collecteur 
de  Clichy,  but  a  new  departure  was  made  in  the  Siphon  de  FOise  in  the  manner  of 
constructing  the  sewer  lining  in  concrete  within  a  steel  plate  casing. 

This  work,  which  forms  a  part  of  the  system  of  sewage  disposal  of  Paris, 
consists  of  a  syphon  tunnel  6  feet  8  inches  in  internal  diameter  which,  starting  from 
a  shaft  on  the  left  bank  of  the  River  Oise,  near  its  junction  with  the  Seine,  at 
Conflans  St.  Honorine,  passes  under  the  river,  and  connects  with  an  outfall  sewer 
towards  Triel,  the  material  tunnelled  through  being  loose  earth,  gravel  and  sand. 

Of  a  total  length  of  4,200  feet  the  part  forming  the  syphon,  919  feet  in  length, 
was  constructed  in  concrete  by  means  of  a  shield,  and  with  compressed  air,  the 
period  during  which  the  shield  was  actually  working  being  from  November,  1897, 
to  December,  1898. 

The  shield  itself  was  of  the  Greathead  type,  closed  in  front  with  a  diaphragm 
containing  doors  somewhat  of  the  pattern  of  the  East  River  Tunnel  Shield,  and 
a  front  hood  with  an  overhang  of  about  3  feet.  Its  length  was,  in  proportion  to 
its  diameter,  much  greater  than  usual,  the  figures  being  16  feet  2  inches  over  all 
to  8  feet  7  inches. 

The  tail  of  the  shield  extended  beyond  the  rearmost  frame  about  6  feet  4  inches, 
due  to  the  necessity  of  securely  holding  the  tunnel  lining  during  the  process  of 
compressing  the  concrete  of  which  it  was  formed.  For  the  same  reason  the  shield 
rams  were  ten  in  number  or  double  the  usual  number  for  an  iron-lined  tunnel  of 
such  small  dimensions. 

The  internal  diameter  of  the  shield  was  4  inches  greater  than  the  external 
diameter  of  the  steel  casing  of  the  tunnel  to  allow  of  this  latter  being  grouted  round 
with  mortar  as  the  shield  was  withdrawn. 

This  space  between  the  two  plates  was  maintained  by  means  of  distance  pieces 
of  steel  fixed  inside  the  tail  of  the  shield,  not  as  in  the  case  ot  the  St.  Clair  and 
Blackwall  Tunnel  Shields  at  the  extremity  of  the  tail,  but  about  3  feet  6  inches 
from  it,  so  that  immediately  the  shield  was  moved  forward,  a  clear  space  was  left 
for  filling  in  the  mortar. 

The  excavation  in  front  was  carried  out  on  the  same  lines  as  in  the  City  and 
South  London  Railway  work  in  water-bearing  gravel.  The  face  was  close  poled,  and 

290 


THE    SHIELD    IN    MASONRY    TUNNELS 


grouf- 


the  sides  also,  and  in  very  bad  ground  the  polings  were  made  very  narrow,  and 
only  removed  one  by  one.  The  joints  were  pugged,  but  no  grouting  appears  to 
have  been  used  with  the  face. 

In  loose  earth  the  crown  was  held  up  by  curved  plates  fitted  under  the  hood 
of  the  shield  (which  hood  was  much  smaller  than  is  usual  in  French  shields),  and 
the  front  edges  of  these  plates  were  driven  into  pockets  made  in  the  face  by  chisels. 

The  hydraulic  pressure  necessary  to  drive  the  shield  rarely  exceeded  20  tons 
per  ram  or  200  tons  in  all,  but  the  rate  of  progress  was  not  very  satisfactory,  even 
allowing  for  the  difficulty  of  arranging  the  work  in  a  tunnel  of  such  small  diameter 
in  bad  ground,  and  with  a  form  of  lining  which  was  composed  of  three  distinct 
pirts  all  having  to  be  worked  separately.  The  mean  progress  was  only  2  feet 

7  inches  per  day,  and  often  only  1  foot  8  inches. 

The  limit  of  air  pressure  was  about  24  pounds  per  square  inch,  the  greatest 

depth   below  water  level 
being  about  43  feet. 

The  airlock  combined 
in  one  structure  the  fea- 
tures of  an  ordinary  hori- 
zontal lock,  combined 
with  a  vertical  chimney 
above  it  with  double 
doors  through  which  con- 
crete could  be  introduced, 
and  having  also  a  side 
shoot  lock  (similar  to 
those  fixed  on  the  Black- 
wall  shield  but  never 
used),  through  which  the 
material  excavated  could 
be  discharged. 

It  is  in  the  perma- 
nent lining  of  the  tunnel, 
however,  that  the  main 
interest  of  this  work  lies. 
Abandoning  the  idea 
both  of  the  temporary 
timber  lining  of  the 
Chagnaud  system,  and  the  immediate  construction  under  and  in  contact  with  the 
skin  of  the  shield  of  the  permanent  masonry  lining  as  in  the  Fougerolle  method, 
the  engineers  erected  within  the  shelter  of  this  shield  two  steel  plate  skins,  the  one 
a  permanent  outside  casing  to  the  concrete  ring,  the  other  a  temporary  centreing 
within  it,  and  the  space  between  these  they  filled  with  concrete  in  lengths  of  1  foot 

8  inches,  compressing  it  by  means  of  the  shield  rams,  which  bore  against  it  and 
effectually  compacted  it,  in  pushing  the  shield  forward. 

The  outside  skin  (see  Fig.  194)  was  composed  of  rings  of  steel  plates  \  inch 
thick  and  20  inches  wide,  each  ring  consisting  of  four  plates  joined  by  bolts  to  each 
other  and  the  adjoining  rings  by  angle  irons  2' 36  inches  by  2'36  inches  by  '23  inch 
riveted  to  the  plates.  The  rings  were  made  to  break  joint. 

Outside  of  this  skin  there  was  left,  as  the  shield  moved  forward,  an  annular 

291 


FIG.   194.     SYPHON  UNDER  THE  RIVER  OISE,  FRANCE. 
Cross  Section  of  Tunnel  Skin,  and  Centreing. 


TUNNEL    SHIELDS 

space  2  inches  wide,  which  was  filled  by  a  mason  "  feeding  "  in  mortar,  which, 
by  the  pressure  of  the  air  in  the  tunnel,  was  carried  in  between  the  steel  ring  and 
the  tail  of  the  shield,  and  effectually  filled  the  space  left  by  the  latter. 

The  inner,  or  centreing  skin,  consisted  of  thicker  plates,  these  being  ?-  of  a 
inch  thick,  made  into  rings  20  inches  wide,  and  6  feet  8  inches  internal  diameter. 

Each  ring  was  composed  of  four  segments,  and  one  key,  the  latter  being  about 
8  inches  wide,  but  the  segments  and  the  keys  were  made  so  that  on  an  average 
ring  a  space  of  about  3  inches  was  left  which  was  closed  by  wedges  covered  with 
^  inch  plates.  In  general,  ten  of  those  rings  were  in  use  at  one  time,  or  in  other 
words,  16  feet  6  inches  of  the  concrete  lining  last  erected  was  kept  supported  by 
them,  and  with  the  rate  of  progression  generally  maintained,  this  meant  that  the 
green  concrete  was  held  up  for  at  least  a  week  after  being  rammed  into  its  place. 


FIG.  195.     SYPHON  UNDER  THE  RIVER  OISE,  FRANCE. 
Longitudinal  Section  of  Tunnel. 

The  method  of  constructing  the  permanent  lining  is  indicated  in  Fig.  196,  in 
which  the  upper  portion  of  the  tail  of  the  shield  and  the  corresponding  part  of 
the  tunnel  lining  is  shown.  A  is  the  tail  plate  of  the  shield,  B  being  one  of  the 
distance  pieces  fixed  within  it  to  ensure  a  space  being  kept  between  the  shield  and 
the  outer  skin  of  the  tunnel  to  allow  of  grout  being  filled  in  outside  the  latter.  At 
the  end  of  the  last  forward  move  of  the  shield  to  its  position  as  shown  in  the  drawing, 
and  the  consequent  erection  of  the  tunnel  lining  within  it,  the  outer  steel  ring  and 
the  centreing  are  complete  to  C,C,  and  the  concrete  between  them  to  D,  the  grouting 
outside  the  outer  ring  being  filled  in  to  E. 

The  tail  of  the  shield  has,  therefore,  an  overhang  of  about  1  foot  over  the 
finished  portion  of  the  tunnel,  and  the  rams  being  drawn  back  into  their  cylinders, 
the  two  outside  rings  F  and  G  are  fixed  in  position,  F  being  grouted  outside  as  far 
as  H,  before  the  second  one  is  erected. 

The  centreing  ring  J  is  then  fixed  and  bolted  to  the  preceding  one,  and  the 

292 


THE    SHIELD    IN    MASONRY    TUNNELS 

space  between  it  and  F  filled  with  concrete,  the  face  of  the  concrete  next  the  shield 
being  temporarily  held  up  during  the  filling  with  shutters  strutted  from  the  frame 
of  the  shield. 

When  the  space  behind  the  ring  J  was  filled,  the  shutters  to  the  concrete  were 
removed  and  the  rams  of  the  shield  applied  to  the  mass  as  in  the  figure.  The 
concrete  was  by  this  means  compressed  as  far  as  H'  and,  being  made  solid,  formed 
a  buttress  against  which  the  shield  advanced  some  20  inches. 

The  rams  were  then  withdrawn,  and  the  centreing  K  erected,  when  the  same 
process  was  repeated. 

The  grout  surrounding  the  tunnel  lining  consisted  of  1  part  of  Portland  cement 
to  3|  parts  of  sand,  and  the  concrete  tunnel  itself  of  1  part  of  Portland  cement, 
3 1  parts  of  sand,  and  4|  parts  of  gravel.  The  rendering  which  was  finally  laid 
over  the  inside  of  the  tunnel  was  J-  inch  thick,  and  consisted  ot  2  parts  of  Portland 
cement  to  3|  of  sand. 

It  was  found  that  it  was  almost  impossible  to  make  the  steel  lining  water- 
tight, by  reason  of  the  difficulty  of  making  the  angle  iron  joints,  fastened  together 
by  bolts,  a  good  fit ;  but  though  this  defect  involved  considerable  extra  expense  in 

.  A 


jKey. 


FIG.   196.     SYPHON  UNDER  THE  RIVER  OISE,  FRANCE. 
Method  of  filling  in  Concrete  Lining. 

maintaining  the  air  pressure,  it  is  probable  that,  by  the  air  drawing  the  cement  of 
the  concrete  to  these  openings,  a  large  quantity  was  held  in  the  concrete,  which,  in 
work  of  this  shape,  would  otherwise  have  been  wasted  in  the  invert.  It  would 
probably  have  been  advantageous  to  have  grouted  up  each  successive  length  or 
ring  of  concrete  by  means  of  a  Greathead  grouting  pan. 

In  the  curved  lengths  of  the  tunnel,  the  necessary  adjustment  of  the  rings  was 
made  by  introducing  lead  packings  of  the  necessary  thickness  between  the  steel 
rings  of  the  lining. 

It  is  clear  that  the  economy  effected  by  the  use  of  a  combined  concrete  and  steel 
plate  lining  instead  of  one  of  cast  iron,  must  almost  entirely  depend  on  the  price  of 
cast  iron  in  the  locality  where  the  work  is  done. 

In  this  case  the  amount  of  excavation  per  yard  forward  was  6|  cubic  yards  as 
against  4|  in  the  case  of  a  tunnel  of  similar  internal  diameter  lined  with  cast  iron, 
while  the  operations  of  erecting  the  steel  and  concrete  lining  were  much  more 
tedious  and  expensive  than  the  same  operations  in  a  cast-iron  tunnel. 

In  any  case  there  can  be  no  doubt  that  where  the  relative  cost  of  the  two 
systems  is  approximately  the  same,  the  ease  with  which  the  cast-iron  tunnel  can 
be  erected  by  unskilled  labour,  and  the  immediate  arrival  of  the  lining  at  its  full 

293 


TUNNEL    SHIELDS 

strength  as  soon  as  erected,  make  the  iron  tunnel  preferable  to  a  composite  masonry 
and  metal  one,  even  if  the  latter  be  constructed  under  the  most  favourable  conditiors 
as  regards  excellence  of  workmanship  and  supervision.  But  it  is  very  doubtfu 
if  at  any  time  a  concrete  lined  tunnel,  built  in  successive  rings  as  one  constructed 
under  a  shield  must  be,  is  ever,  as  regards  strength,  anything  like  as  strong  as  an 
ordinary  concrete  structure  of  similar  dimensions,  but  constructed  on  conditions 
which  obtain  in  ordinary  engineering  work  in  concrete.  It  is  very  questionable 
whether  the  successive  rings  of  concrete  really  bond  together,  and  it  is  always 
possible,  in  the  case  of  trouble  with  the  shield,  or  delay  by  any  other  cause  in 
compressing  the  concrete  in  position,  that  when  this  is  done,  crystallization  of  the 
cement  has  already  occurred,  and  consequently  the  actual  strength  of  the  mass  is 
far  below  its  presumed  quality.  A  free  use  of  grout  sprayed  over  the  face  of  the 
finished  work  so  that  the  escaping  air  from  the  pressure  chamber  would  force  it 
into  the  interstices  of  the  concrete  would,  no  doubt,  do  much  towards  consolidating 
it,  but  this  at  best  is  a  palliative,  and  concrete  fortified  in  this  way  is  not  the  same 
as  concrete  properly  set  once  and  for  all. 

The  Paris  Extension  of  the  Orleans  Railway  (1898) 

Until  the  year  1898,  the  Paris  Terminus  of  the  Orleans  Railway  Company  was 
in  the  Place  Walhubert,  close  to  the  Pont  d'Austerlitz,  but  the  inconvenience  of  a 
terminus  so  far  from  the  central  part  of  the  city  induced  the  company  in  that  year 
to  extend  their  line  along  the  left  bank  of  the  Seine  to  the  Quai  d'Orsay,  where, 
near  to  the  southern  end  of  the  Pont  de  Solferino,  a  new  terminus  was  built.  The 
length  of  this  extension  is  about  2|  miles,  of  which  distance  the  whole,  with  the 
exception  of  the  portion  between  the  Pont  d'Austerlitz  and  the  Pont  Sully,  is  in 
tunnel,  and,  as  the  level  of  the  quays  along  the  whole  length  is  only  a  few  metres 

above  the  ordinary  level  of  the 
river,  the  invert  of  the  tunnel  is 
constructed  in  water-logged  mate- 
rial (see  Fig.  222). 

The  section  of  the  tunnel  is, 
for  the  greater  portion  of  the 
length  of  the  ordinary  double 
line  masonry  tunnel  type,  29  feet 
6  inches  wide,  and  with  a  headway 
above  the  rails  of  15  feet  7  inches. 
The  roof  is  elliptical  in  shape, 
2  feet  thick  at  the  crown,  and  rests 
on  two  side  walls  \^  ith  a  thickness 
of  about  2  feet  7  inches,  the  in- 
vert being  1  foot  9  inches  thick. 

The  masonry  was  of  concrete 
blocks,  or  of  limestone,1  set  in 
cement  mortar.  The  concrete 

blocks  were  in  the  proportion  of  4'5  cwt.  of  Portland  cement  to  21  cubic  feet  of 
stone,  and  14  cubic  feet  of  sand. 

i  Limestone  was  used  with  the  shield  employed  between  the  Quai  d'Orsay  and  the  Place 
St.  Michel. 


FIG.   197.     ORLEANS  E  AIL  WAY,  EXTENSION,  PARIS. 

Cross  Section  of  Tunnel  between  the  Place  St.  Michel 

and  the  Quai  d'Orsay. 


294 


THE    SHIELD    IN    MASONRY    TUNNELS 

The  portion  of  the  tunnel,  however,  between  the  Place  St.  Michel  and  the 
terminus  was  constructed  with  the  view  of  constructing  at  a  later  date  a  second 
tunnel  alongside  the  first,  and  to  do  this  the  side  wall  on  that  side  (and  the  haunch 
of  the  arch  above  it)  was  constructed  of  extra  strength,  thus  making  the  cross 
section  of  the  tunnels  un-symmetrical  (see  Fig.  197). 

This  portion  of  the  tunnel  was  only  26  feet  3  inches  wide,  with  a  headway 
at  the  centre  over  the  rails  of  17  feet  3  inches,  and  like  the  other  part  was  built 
of  moulded  concrete  blocks  set  in  cement  mortar. 

In  both  sections,  the  extrados  of  the  arch  was  covered  with  cement  ren- 
dering nearly  3  inches  thick,  and  the  intrados,  side  walls  and  dish  of  the  invert  with 


FIG.   198.     ORLEANS  RAILWAY  EXTENSION,  PARIS. 
Method  of  driving  Side  Headings  in  advance  of  the  Chagnaud  Shield. 


a  thickness  of  H  inches,  while  a  damp  course  of  the  same  material  was  made  in 
the  middle  of  the  invert,  and  carried  up  in  the  side  walls,  nearly  to  the  springing 
line  (see  Figs.  197  and  198.) 

The  methods  employed  for  the  construction  of  these  tunnels  varied  somewhat, 
but  two  lengths,  namely,  from  the  Ponte  de  Sully  to  the  Petit  Pont  and  from  the 
Place  St.  Michel  to  the  terminus  under  the  Quai  d'Orsay,  were  built  with  the  aid 
of  roof  shields.  These  were  both  the  designs  of  M.  Chagnaud,  and  in  general 
followed  the  lines  of  the  one  he  had  so  successfully  used  in  the  Collecteur  de  Clichy. 

One  very  important  variation,  however,  was  made  from  the  earlier  practice  ; 
the  side  walls  were  first  built  in  headings  driven  and  timbered  in  the  ordinary 

205 


TUNNEL    SHIELDS 

manner,  and  on  these  sidewalls,  the  shield  subsequently  was  rolled  forward,  the 
masonry  of  the  walls  forming  solid  bases  on  which  the  roller  paths  could  be  laid 
without  risk  of  settlement. 

The  various  stages  in  this  method  of  working  can  be  understood  from  Figs. 
198  to  201.  The  first  stage  consisted  in  opening  out  two  side  galleries  or  headings  on 
either  side  of  the  tunnel,  timbered  in  the  ordinary  way  with  head  and  side  trees,  and 
close  poled  in  the  top  and  outer  side.  These  headings  were  made  sufficiently  high 
to  permit  of  the  side  walls  being  built  up  to  a  certain  height,  leaving  space  for  the 
laying  of  the  platforms  on  which  the  shield  subsequently  moved. 

The  second  stage  comprised  the  construction  of  the  side  walls  within  these 
headings,  and  the  transfer  of  the  weight  of  the  roof  from  the  outside  side  trees  to 
the  new  walls. 

The  operations  of  these  two  stages  were  kept  well  ahead  of  the  sh'eld  which 
formed  the  third  stage  of  the  work,  so  that  the  side  walls  had  time  to  become  set 
before  the  weight  of  the  shield  was  put  on  them. 

The  shield  in  its  advance  removed  all  the  upper  part  of  the  excavation,  making, 
as  it  went  on,  the  masonry  tunnel  complete,  as  regarded  the  arch  and  side  walls,  and 
leaving  behind  to  be  excavated  only  the  dumpling  between  the  two  headings 
originally  driven,  and  the  space  for  the  invert. 

The  fourth  and  last  stage  included  the  removal  of  the  central  dumpling,  and 
the  construction  of  the  invert,  which  was  built  in  short  lengths. 

In  some  cases  the  side  headings  driven  were  made  large  enough  to  allow  of 
the  construction  of  3  or  4  feet  of  invert,  in  addition  to  the  side  wall,  in  others 
the  side  walls  alone  were  built  (both  forms  of  construction  are  shown  in  Fig. 
198). 

The  material  tunnelled  through  consisted  almost  entirely  of  made  ground, 
and  of  old  masonry  walls  and  foundations.  These  latter  gave,  naturally,  great 
trouble,  and  in  some  cases  necessitated  the  sinking  of  trenches  from  the  street 
level. 

Generally  speaking,  however,  the  tunnel  work,  which  in  some  cases  was  only 
1  foot  3  inches  below  the  quay  level,  was  carried  on  without  interruption  of  the 
road  traffic  above ;  when,  however,  the  cover  was  as  little  as  2  feet,  the  movement 
of  the  shield  usually  produced  in  advance  of  the  cutting  edge  an  undulation  in  the 
road  which,  in  one  case,  reached  the  height  of  3  feet,  and  in  such  places  the  tunnel 
was  built  by  cut  and  cover  work,  the  shield  being  simply  pushed  forward  into  the 
solid  beyond. 

The  shield  employed  between  the  Pont  de  Sully  and  the  Petit  Pont  was  designed 
by  M.  Chagnaud  on  the  same  lines  as  the  shield  of  the  Collecteur  de  Clichy  (extra 
muros),  but  with  a  modification  of  the  arrangement  of  the  centres  and  the  polings 
and  lagging  above  them,  designed  to  obviate  the  necessity  for  using  lagging  by 
means  of  a  system  of  girders  and  moveable  timbers  which  formed  a  kind  of  pro- 
longation of  the  shield,  and  though  capable  of  movement  relatively  to  it  were  not 
rigidly  attached. 

The  scheme  proved  a  failure  and,  the  new  centres  being  removed,  the  section 
was  finished  by  employing  the  methods  of  the  earlier  shield.1 

The  second  shield  employed  was,  however,  more  successful,  the  general  arrange- 
ments for  erecting  the  masonry  tunnel  being  on  the  lines  of  the  system  employed 

1  For  description  and  plate  of  this  machine  see  Genie  Civile  of  February  11,  1899.  See 
also  Philippe's  Le  Bouclier,  pp.  64-83. 

296 


THE    SHIELD    IN    MASONRY    TUNNELS 

on  the  Collecteur  de  Clichy  intra  muros,  the  masonry  being  laid  under  the  protection 
of  the  shield  itself. 

This  shield  is  shown  in  Figs.  199,  200  and  201,  and  is  singular  in  being  made 
dissymmetrical  to  suit  the  shape  of  the  tunnel  on  the  length  from  the  Quai  d'Orsay 
to  the  Place  St.  Michel.  It  is  a  roof  shield  or  "  carapace,"  and  consists  essentially 
of  a  plate  skin  fixed  in  a  frame  formed  by  two  semi-elliptical  girders  E,  E,  about  5 
feet  1|  inches  apart,  centre  to  centre,  and  rigidly  framed  together  by  numerous 
plate  bracings  F,  F,  which  again  are  secured  to  an  internal  skin. 

They  rest  on  the  cellular  girders  G,  G,  the  bottom  flanges  of  which  form  the 
tables  beneath  which  are  laid  the  rollers  H,  H.  They  are  prevented  from  spreading 


of  3 treat 


FIG.   199.     ORLEANS  RAILWAY  EXTENSION,  PARIS. 
The  Chagnaud  Shield  :    Longitudinal  Section,  showing  method  of  working. 


by  the  horizontal  girders  J,  J,  which  support  the  working  platform  of  the  shield,  and 
carry  at  their  centres  king  posts  K,  K,  supporting  the  crowns  of  the  main  girders. 
They  also  are  braced  together  by  smaller  girders  L,  L. 

The  skin  itself  was  f  inch  thick,  the  front  of  the  cutting  edge  and  all  of  the 
tail  being  ot  double  thickness.  Its  length  over  all  was  23  feet,  the  cutting  edge 
overhanging  8  feet  2  inches  beyond  the  front  one,  and  the  tail  extending  9  feet 
8  inches  behind  the  rear  one  of  the  girders  E,  E.  The  horizontal  width  of  the  shield 
was  32  feet,  and  its  height  from  the  underside  of  the  girders  G,  G,  to  the  extrados 
of  the  skin  11  feet  8  inches. 

The  girders  E,  E,  were  1  foot  10  inches  in  depth  of  web,  with  angle  irons  4  inches 
by  4  inches  by  £  inch.  Rivetted  to  the  lower  angles  was  a  plate  M ,  which  formed, 
so  to  speak,  the  lower  flange  of  a  box  girder,  of  which  the  other  flange  was  the 

297 


TUNNEL    SHIELDS 

skin,  and  the  two  girders  the  vertical  webs.  This  plate  M  projected  some  distance 
in  advance  of  the  front  girder  E,  thus  giving  additional  support  to  the  gussets  of 
the  cutting  edge  (see  Fig.  199). 

The  skin  of  the  shield,  which  both  before  and  behind  was  of  less  extent  at  the 
base  than  at  the  crown,  and  the  two  girders  E,  E,  were  carried  on  the  box  girders 
O,  G,  which  extend  beyond  the  main  girders  about  2  foot  9  inches  below  the  cutting 
edge,  and  1  foot  9  inches  under  the  tail,  of  the  shield.  These  girders  were  2  feet 
deep,  and  considering  that  the  entire  weight  of  the  shield  rested  on  them,  and  that 


FIG.  200.     ORLEANS  RAILWAY  EXTENSION,  PARIS. 
The  Chagnaud  Shield  :    Cross  Sections  A,  B  and  C,  D,  Fig.   199. 

they  had  to  meet  any  unequal  loading  caused  by  settlement  of  the  masonry  bed  in 
which  the  machine  moved,  they  do  not  appear  too  strong,  although  it  must  be  said 
they  are  much  more  satisfactory  than  the  similarly  placed  single  web  girders  in  the 
Chagnaud  shield  of  the  Clichy  "  extra  muros  "  sewer,  in  which  there  was  no  masonry 
bearing  for  the  shield  at  all. 

The  overhanging  roof  of  the  shield  was  stiffened,  both  fore  and  aft,  by  brackets 
N,  N,  arranged  in  line  with  the  gussets  F,  F,  between  the  main  girders  ;  these  were, 
as  will  be  seen  from  the  section  A,  B,  and  C,  D,  Fig.  200,  arranged  in  couples,  under 

298 


THE    SHIELD    IN    MASONRY    TUNNELS 

each  of  which  the  inside  plate  or  skin  M  was  thickened  by  an  extra  plate  0  forming 
a  base  to  which  was  bolted  one  of  the  rams  P. 

At  the  crown  of  the  shield,  the  distance  between  these  brackets  was  nowhere 
more  than  2  feet,  and  it  is  due  to  the  strength  thus  given  to  the  shield,  that  it 
arrived  at  the  end  of  its  work  in  good  order,  without  any  buckling,  a  condition  of 
things  not  usual  in  machines  with  plate-cutting  edges. 

The  total  weight  of  the  shield  including  the  rams  and  their  fittings  was  about 
100  tons. 

The  rams,  ten  in  number,  were,  as  at  Clichy,  but,  contrary  to  the  usual  custom, 
placed  below  the  main  girders  instead  of  within  them,  and  as  already  described  were 


Half  dectw 


~~^*^> 

/-TStj-'or  .T,.-»->,t-»,,~  VJf—  OR 

}{aLf  Section,  atrearof 


FIG.  201.     OELEANS  RAILWAY  EXTENSION,  PARIS. 
The  Chagnaud  Shield  :    Cross  Sections. 


bolted  to  the  gussets  F,  F.  This  arrangement  puts  a  very  heavy  strain  on  the 
bolts,  and  on  the  frame  of  the  shield,  but  has  the  advantage  that,  the  position  of 
the  rams  being  fixed,  by  the  fact  that  they  must  bear  on  the  centres  carrying  the 
masonry  of  the  tunnel,  the  reduction  of  the  depth  of  the  girders  E,  E,  sufficiently  to 
clear  the  rams  greatly  increases  the  working  space  in  the  shield. 

The  stroke  of  the  rams  was  4  feet  3  inches,  and  the  diameter  of  the  pistons 
9J  inches,  the  hydraulic  pressure  employed  being  about,  on  the  average,   1,000 

299 


TUNNEL    SHIELDS 

pounds  to  the  square  inch.  This  pressure  was  supplied  by  an  electrically  driven 
pump  fixed  on  the  stage  in  the  shield. 

The  rams  were  constructed  with  a  central  draw-back  cylinder  similar  to  those 
used  on  the  Great  Northern  and  City  Railway  and  in  the  Holborn  Tunnels,  and 
shown  in  detail  in  Fig.  76. 

The  rollers  H,  H,  on  which  the  shield  advanced,  were  7  inches  in  diameter,  and 
rested  on  elm  sole  pieces  E,  E,  having  their  upper  surface  protected  by  a  f  inch  plate. 
These  sole  pieces  were  laid  on  the  previously  constructed  masonry  side  walls  which 
afforded  a  rigid  base  for  the  operations  of  the  shield. 

In  addition  to  these  horizontal  rollers  two  vertical  ones  H',  H' ,  were  fixed 
on  either  side  of  the  shield,  and  so  placed  as  to  bear  against  sole  pieces  fixed  against 
the  face  of  the  side  walls.  These  assisted  materially  in  guiding  the  shield,  by 
providing,  so  to  say,  a  wheel  base  4  feet  1  inch  long  rolling  on  each  side  wall.  A 
longer  distance  between  the  rollers  might  have  been  arranged  with  advantage,  as 
no  curve  traversed  by  the  shield  was  less  than  1,000  feet  radius,  and  an  increased 
length  between  them  would  have  increased  the  guiding  power  of  the  rollers,  with- 
out increasing  the  difficulty  of  driving  the  shield. 

The  working  of  the  shield  was  on  similar  lines  to  that  of  the  shields  described 
earlier  in  this  chapter.  As  the  shield  advanced  by  pushing  against  the  centres 
supporting  the  tunnel  behind,  the  masonry  arch  was  built  up  in  steps,  the  tail  of 
the  shield  being  long  enough  to  admit  of  the  erection  of  three  centres  3  feet  11 
inches  apart. 

Seven  masons  were  employed,  three  working  on  either  side  between  the  three 
centres,  while  the  fourth  keyed  in  the  arch  over  the  last  of  the  three.  The  most 
troublesome  part  of  the  work  was  the  packing  securely  with  mortar  the  spaces 
left  by  the  tail  plates  of  the  shield,  and  the  necessity  of  doing  this  with  care  some- 
what retarded  the  rate  of  progress,  or  at  any  rate  made  it  more  convenient  to 
advance  the  shield,  not  in  lengths  of  4  feet  to  suit  the  distance  between  the  centres. 
but  in  lengths  of  15  to  16  inches  at  a  time. 

The  mortar  grouting  was  the  more  important,  as  in  general  the  masonry  was 
not  built  tight  to  the  skin  of  the  shield,  but  with  about  1  inch  clear  all  round. 

In  constructing  the  roof  of  the  tunnel  under  shield,  water  was  not  met  with, 
the  level  of  the  river  water  being  well  below  the  springing  line  of  the  arch,  and 
indeed  it  is  not  easy  to  see  how  the  work  of  constructing  the  permanent  masonry 
lining  could  have  been  carried  out  had  the  material  surrounding  the  shield  been 
waterlogged. 

The  excavation  in  front  of  the  shield  and  the  filling  of  the  skips  was  performed 
by  a  gang  of  ten  men,  and  was  much  facilitated  by  the  existence  of  an  old  disused 
sewer  which  lay  along  the  line  of  the  tunnel  and  along  which  the  skips  of  spoil 
were  sent  forward  and  discharged  into  barges  at  a  convenient  point,  thus  avoiding 
all  interference  with  the  service  of  skips  delivering  stone  and  mortar,  etc.,  for  the 
construction  of  the  tunnel. 

The  average  rate  of  progress  was  a  little  less  than  10  feet  per  day. 

The  centres  were  forty  in  number,  and  weighed  with  their  cast-iron  struts, 
etc.,  about  16  cwt.  each. 

Each  centre  a,  a,  Fig.  201,  consisted  of  a  web,  1  foot  3  inches  deep  and 
J  inch  thick,  stiffened  with  two  angle  irons  3£  inches  by  3J  inches  by  f  inch  rivetted 
on  at  top  and  bottom.  For  convenience  of  handling,  each  centre  was  made  in 
two  halves,  bolted  together  at  the  crown.  When  in  position,  it  was  wedged  up 

300 


THE    SHIELD    IN    MASONRY    TUNNELS 

at  the  ends  on  props,  b,  b,  and  supported  also  by  rakers  c,  c,  the  effect  of  which  was 
to  maintain,  better  than  had  so  far  been  the  case,  the  proper  shape  of  the  centres. 
Between  each  pair  of  centres  were  ten  cast-iron  distance  .pieces,  d,  d,  d,  which  were 
axial  with  the  rams  of  the  shield,  the  thrust  of  which  they  distributed  along  the 
whole  number  of  centres.  It  was  found,  however  carefully  these  distance  pieces 
were  fitted,  that  a  movement  took  place  among  the  centres,  when  the  pressure 
of  the  shield  was  applied,  to  the  extent  of  about  f  inch.1 

Instead  of  the  usual  lagging  on  the  centres,  small  frames  e,  e,  e,  made  of 
strips  of  wood,  were  used  (see  Fig.  201).  These  frames  were  about  3  feet  4  inches 
long  and  9  inches  wide,  and  instead  of  being  like  ordinary  lagging  laid  parallel 
to  the  centre  line  of  the  tunnel,  were  laid  with  their  longer  sides  parallel  to  the 
centres,  their  ends  being  supported  on  the  cast  metal  flanges  of  the  distance 
pieces  d,  d,  d. 

"  The  employment  of  these  frames  gave  good  results,  enabling  the  masons  to 
keep  always  close  to  their  work,  and  consequently  giving  them  greater  freedom  of 
movement." 

M.  Rene  Philippe,  from  whom  the  previous  lines  are  quoted,  makes,  in  com- 
menting on  the  general  progress  of  the  work,  the  following  observations2  : — 

Generally  the  roadway  (above)  was  made  as  the  shield  went  on,  for  the  caulking  by  grout, 
although  carefully  executed,  could  not  fill  the  hollow  lift  by  the  removal  of  stone  blocks  and 
other  obstacles. 

When  the  rate  of  excavation  was  slow  the  masons  overtook  the  miners,  and  this  in  spite 
of  the  fact  that  their  numbers  were  reduced  from  eight  to  six,  and  that  they  were  employed 
in  moving  the  centres  ;  but  later,  the  masonry  kept  back  the  miners,  to  the  extent  of  making 
them  lose  two  "pushes"  of  the  shield,  or  2  feet  8  inches,  in  an  eleven-hour  shift. 

In  loose  ground  the  nose  of  the  cutting  edge  was  extended  by  means  of  wood  polings  fitted 
inside  the  hood,  and  between  it  and  an  angle  iron  fixed  inside  it,  and  about  1  foot  behind  the 
cutting  edge. 

In  consequence  of  the  tail  of  the  shield  failing  on  several  occasions,  it  was  found  necessary 
to  raise  up  the  shield  in  order  to  maintain  the  masonry  arch  at  its  proper  level  above  the  rails. 
The  sole  pieces  (R,  R,  in  Fig.  200)  were  in  consequence  thickened,  with  the  double  inconveni- 
ence that,  in  the  first  place,  the  thicker  pine  sole  pieces  were  compressed  under  the  weight  of 
the  shield,  thus  causing  it  to  undulate  in  its  advance,  and,  secondly,  the  vertical  rollers  being 
left  with  a  less  bearing  surface,  failed  to  act  as  guides  to  the  machine. 

The  value  of  the  lateral  bearings  was  here  clearly  shown,  the  bearing  plate  fitted  on  the 
face  of  the  side  wall  showing  the  effect  of  the  vertical  roller  bearing  on  it,  while  no  sign  of  failure 
appeared  in  the  wall  itself. 

Under  the  pressure  of  the  shield  rams,  the  cast  iron  distance  pieces  were  compressed,  and 
the  centres  moved  backwards,  sliding  under  the  arch  of  the  tunnel  :  the  extent  of  this  move- 
ment of  the  centres  being  shown  by  numerous  observations  to  amount  to  from  14  to  18  milli- 
metres each  "  push  "  ;  and  when  the  rams  were  drawn  back,  it  was  noticed  that  the  centres, 
in  springing  back  to  their  original  position,  were  likely  to  draw  the  more  recently  constructed 
masonry  with  them  ;  and  in  consequence  the  -system  was  adopted  of  wedging  the  lagging 
frames,  e,  e,  e,  against  the  back  girder,  E,  of  the  shield  (at  the  end  of  the  stroke),  by  means  of 
eight  chogs  placed  alongside  the  cast-iron  distance  pieces  ;  with  the  result  that  no  movement 
was  observed  among  the  lagging  frames,  nor  fissures  in  the  masonry  lining. 

This  shield  seems  to  have  been  a  great  advance  on  the  previous  machines 
of  M.  Chagnaud.  In  its  general  construction  it  appears  to  have  been  well-propor- 
tioned for  its  work,  and  in  the  method  employed  of  constructing  in  advance  the 
side  walls  of  the  tunnel  as  bases  on  which  the  shield  could  advance,  there  is  no 

1  Philippe's  Le  Bouclier,  p.  92.  2  Philippe's  Le  Bouclier,  p.  92. 

301 


TUNNEL    SHIELDS 

doubt  that  its  designer  has  found  the  best  method  of  driving  a  shield  for  the 
construction  of  shallow  tunnels  in  loose  or  compressible  material  when  water  is 
absent. 

In  details,  the  use  of  vertical  rollers  bearing  on  the  faces  of  the  side  walls  already 
constructed,  appears  to  be  justified  by  the  results,  and  the  substitution  of  small 
framed  laggings  for  the  usual  horizontal  strips  on  the  centres  proved  a  useful 
innovation. 


302 


Chapter    IX 
THE  SHIELD  IN  MASONRY  TUNNELS  (continued) 

THE  TREMONT  STREET  TUNNEL,  BOSTON,  U.S.A. — WORK  COMMENCED  WITHOUT  A  SHIELD — 
A  ROOF  SHIELD  DECIDED  ON — METHOD  OF  WORK — DETAILS  OF  CONSTRUCTING  THE  SIDE 
WALLS — DETAILS  OF  THE  SHIELD — THE  SLIDING  SHOES — CAST  IRON  BARS  BUILT  IN  THE 
BRICK  ARCH  TO  RECEIVE  THE  THRUST  OF  THE  SHIELD  RAMS — RATE  OF  PROGRESS — THE 
BOSTON  (U.S.A.)  HARBOUR  TUNNEL — CONDITIONS  OF  COMPRESSED  AIR  WORK — DETAILS 
OF  THE  SHIELD — METHOD  OF  WORKING — RATE  OF  PROGRESS — THE  METROPOLITAN  RAIL- 
WAY OF  PARIS — COMPARATIVELY  LIMITED  EMPLOYMENT  OF  SHIELDS — METHODS  OF  SHIELD 
WORK  ADOPTED — SECTIONS  IN  WHICH  SHIELDS  WERE  EMPLOYED — THE  CHAMPIGNEUL 
SHIELDS — DETAILS  OF  THE  SHIELD — CENTRAL  ADVANCE  HEADING  USED — METHOD  OF 
WORKING — CENTRES  FOR  MASONRY — RATE  OF  PROGRESS — INTERRUPTION  OF  STREET 
TRAFFIC  ABOVE — GENERAL  REMARKS  ON  THE  CHAMPIGNEUL  SHIELD — THE  LAMARRE 
SHIELDS — DETAILS  OF  THE  SHIELD — TIMBER  CENTRES — UNSATISFACTORY  RESULTS  OF 
WORKING — THE  DIEUDONNAT  SHIELDS  AND  THE  WEBER  SHIELDS — GENERAL  REMARKS 
ON  THE  METROPOLITAN  RAILWAY  SHIELDS 

The  Boston  Underground  Railway  (1897) 

TOURING  the  construction  of  this  railway,  which  was  commenced  in  1894,  and 
-1—'  is  now  approaching  completion,  roof  shields  were  employed  on  two  sections 
of  the  line  for  building  a  double  line  tunnel  of  concrete  and  brickwork,  and  achieved 
a  marked  success.  In  the  first  section  carried  out  in  this  manner  under  Tremont 
Street  in  1897,  at  a  depth  of  from  10  to  14  feet  from  the  roadway  to  the  extrados 
of  the  arch,  the  street  traffic  was  but  little  interfered  with,  and  the  rate  of  progress 
of  the  work  was  satisfactory. 

In  the  second  length,  where  shields  were  employed,  a  similar  tunnel,  but  entirely 
of  concrete,  was  driven  under  Boston  Harbour,  the  minimum  cover  being  about 
18  feet  and  the  maximum  height  of  water  90  feet  above  the  invert  of  the  tunnel. 
Compressed  air  was  employed  in  this  part  of  the  work,  which  was  commenced  in 
1900  and  is  now  finished. 

The  Tremont  Street  Tunnel 

The  Tremont  Street  Tunnel  (1897)  should,  if  strict  chronological  order  were 
adhered  to,  be  described  between  the  Clichy  Sewer  of  1895  and  the  Orleans  Railway 
of  1898.  It  shows,  however,  with  the  Harbour  Tunnel,  constructed  under  the 
same  engineer,  Mr.  H.  A.  Carson,  such  an  advance  on  the  Paris  undertakings, 
and  the  two  tunnels  have  so  many  features  in  common,  that  they  are  brought 
together  in  the  same  chapter,  although  the  one  was  built  some  years  later  than  the 
other  (see  Figs.  202,  203). 

303 


TUNNEL    SHIELDS 

The  railway  under  this  street  consists,  at  its  south  end  at  the  junction  with 
Park  Street,  of  two  separate  single  line  tunnels  which  unite  in  a  bellmouth  in  Tremont 
Street,  and  this  in  turn  becomes  a  double  line  tunnel  with  a  width  at  springing  of 
23  feet,  and  height  from  invert  to  crown  of  arch  of  17  feet  9  inches.  This  section 


FIG.  202.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 

Plan  showing  position  of  the  Sections  constructed  under  Shield  in  Tremont  Street  and  under  the 
Harbour.     The  full  Black  lines  indicate  the  lengths  of  Shield  work. 

of  tunnel  extends  the  full  length  of  Tremont  Street  to  its  junction  at  its  north  end 
with  Scollay  Square. 

It  was  originally  specified  that  the  tunnel  should  be  built  for  a  short  distance 
at  either  end  of  the  street  by  the  "  slice  "  method,  and  the  remainder  by  "an 


FIG.  203.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
Longitudinal  Sections  showing  Tremont  Street  and  Harbour  Tunnels. 

acceptable  mode  of  tunnelling,"  the  provisions  of  the  contract  as  first  awarded 
being  such  as  practically  gave  the  public  the  enjoyment  of  the  entire  roadway 
and  footpaths  during  the  day,  and  even  at  night  gave  the  necessary  area  for  the 
circulation  of  wheeled  and  foot  traffic. 

304 


THE    SHIELD    IN    MASONRY    TUNNELS 

The  contractor  for  the  Tremont  Street  tunnel,  which  was  known  as  "  Section 
6  "  of  the  railway,  commenced  operations  by  excavating  two  "  slices  "  at  the 
north  end  of  his  contract  about  24  feet  apart,  and  then,  in  order  to  demonstrate 
the  feasibility  of  the  method  he  proposed  to  follow  for  the  remainder,  commenced 
to  connect  them  by  tunnelling  (as  distinguished  from  the  "  cut  and  cover  "  of  the 
"  slice  "  work). 

His  method  was  to  drive  in  the  first  place  small  headings  on  the  lines  of  the 
side  walls,  but  only  one-half  their  height,  to  build  in  these  the  lower  portion  of  the 
concrete  walls,  and  then  to  drive  a  second  heading  over 
each  of  the  first  to  obtain  the  full  height  of  the  side  walls. 
He  then  drove  a  centre  heading  on   the  line  of  the  axis  of 
the  tunnel,  about  10  feet  wide  and  6  feet  deep,  roofed  with 
squared  crown  bars  about  12  feet  long  which  were  placed 
to  clear  the  proposed  arch. 

This  heading  was  subsequently  widened  out  by  driving 
laggings  in  either  side,  the  whole  of  the  roof  so  made  being   FIG.  204.    UNDERGROUND 
supported  on  timbers  resting  on  foot-blocks.     At  the  same    RAILWAY,  BOSTON,  U.S.A. 

, .  .  Section  of  Tremont  Street 

time  another  similar  heading  with  crown  bars  was  driven  Tunnel, 

by  the  engineers  of  the  railway   in   the   adjoining   length. 

The  results  of  the  two  experimental  headings  were  alike  :  the  sandy  clay,  sand 
and  gravel,  containing  some  boulders,  forming  the  natural  material  below  the 
street,  and  doubtless  disturbed  by  the  street  and  building  excavations  of  a  hun- 
dred and  fifty  years,  showed  signs  of  settlement  immediately  the  central  headings 
were  made  ;  and  the  movement  naturally  increased  as  the  excavation  was  widened 
out.  In  consequence  the  contractor  was  notified  that  the  crown  bar  system  of 
tunnelling  was  unsatisfactory,  and  shortly  after  the  Commissioners  directing  the 
work  of  the  railway  took  over  the  construction  of  the  section,  and  finished  it  with- 
out the  intervention  of  a  contractor. 

Mr.  Carson,  the  Commissioners'  Chief  Engineer,  decided  to  employ  a  shield 
for  the  construction  of  the  length  of  tunnel  between  the  "  bell  mouth  "  at  the  south 
end  of  Tremont  Street,  and  the  junction  of  this  street  with  School  Street.  North 
of  School  Street  he  continued  to  employ  the  "  slice  "  method,  but  the  methods 
used  in  this  work  do  not  come  within  the  scope  of  this  book. 

Where  the  roof  shield  was  used,  the  method  of  working,  already  described  as 
in  use  in  the  extension  of  the  Orleans  Railway  in  Paris,  was  in  its  main  features 
adopted  with  one  new  and  most  important  feature,  though  in  point  of  time  it  must 
be  remembered  that  Mr.  Carson's  work  preceded  the  Paris  undertaking.  In  the 
first  place  (see  Pig.  205)  the  side  walls  were  first  built  in  concrete  in  small  drifts 
timbered  with  head  and  side  trees,  and  the  roof  and  nearly  the  whole  of  the  sides 
close  poled.  For  a  length  of  132  feet  at  the  south  end  of  the  total  length  of  550 
feet  built  under  shield,  these  side  walls  were  built  in  double  tunnel,  as  described 
above,  but  for  the  remainder  single  headings  of  sufficient  size  to  admit  of  the  whole 
side  wall  being  built  in  one  operation  were  driven. 

In  both  cases,  after  the  headings  were  driven,  and  before  starting  the  masonry 
of  the  side  walls  proper,  the  spaces  between  the  timber  settings  were  filled  with 
concrete,  flush  with  the  inside  face  of  the  frames,  all  polings  being  removed  as  the 
concrete  was  put  in.  In  this  way  a  close  joint  between  the  concrete  and  the  sur- 
rounding earth  was  insured,  and  the  possibility  of  any  pockets  or  cavities  behind  the 
wall  avoided. 

305  x 


FIG.  205.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
of  Tunnelling  under  Tremont  Street. 


306 


FIG.  206.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
Method  of  Tunnelling  under  Tremont  Street,    j 


307 


TUNNEL    SHIELDS 


The  inside  surface  of  this  concrete  was  then  plastered  with  Portland  cement, 
which  in  turn  was  treated  with  asphalt,  to  make  a  vertical  water-tight  joint. 
Against  this  face  again  was  built  the  true  side  wall  of  the  tunnel  (see  Fig.  205),  which 


t    P5    S 


11 


was  carried  up  to  the  springing  line  of  the  arch,  and  on  its  top  was  fixed  the  roadway 
in  which  the  shield  subsequently  travelled.  This  consisted  of  two  rolled  joists 
(Figs.  206  and  208)  10  inches  high  with  5-inch  flanges,  framed  together  and 
having  rivetted  to  their  bottom  flanges  a  plate  15  inches  wide  and  1  inch  thick. 

308 


THE    SHIELD    IN    MASONRY    TUNNELS 


These  were  embedded  nearly  their  full  depth  in  each  wall,  and  made  perfectly 
rigid  bearing  surfaces  for  the  shoes  of  the  shield. 

The  shield  weighed  about  22  tons  and  cost  about  £1,200.  In  general  arrange- 
ment it  differed  but  little  from  the  roof  shields  previously  considered,  the  main 
structural  difference  being  in  the  more  satisfactory  proportion  of  its  base  to  the 
length  and  height  of  the  machine  (see  Figs.  207,  208,  209). 1 

The  length  of  the  skin  of  the  shield  was  12  feet  long  and  1  inch  thick,  the  main 
ribs  E  and  F  supporting  it  being  4  feet  apart.  The  front  hood  had  a  projection 
of  4  feet,  the  skin  being  bevelled  off  so  that  it  only  extended  2  feet  below  the  base 
of  rib  E.  Behind,  the  tail  overhung  4  feet,  an  extra  plate  C,  however,  2  feet  wide, 
extending  2  feet  beyond 
the  tail  proper,  to  assist 
in  protecting  the  keying 
in  of  the  tunnel  arch. 

The  length  of  the  base 
was  over  one-third  of  the 
total  length  of  the  shield 
roof. 

The  width  of  the 
shield  over  all  was  29  feet 
4  inches,  and  its  height 
over  all  was  8  feet  7J 
inches. 

The  length  of  the 
shield,  therefore,  was  much 
less  in  proportion  to  its 
width,  and  the  width  of 
the  base  greater  in  pro- 
portion to  its  length,  and 
to  its  height  greater  than 
in  any  of  the  Paris  shields.  The  first  modification  of  the  French  pattern 
ensured  greater  ease  of  direction,  the  second  greater  stability.  The  ribs  E 
and  F  were  made  each  3  feet  8  inches  deep,  the  web  being  £  inch  thick  for  the 
greater  part  of  its  length  and  f  inch  thick  at  the  ends  over  the  bearings.  The 
flanges  consisted  of  plates  12|  inches  wide  by  f  inch  thick  at  the  centre,  and  12^ 
inches  wide  by  -|  inch  thick  at  the  ends  to  correspond.  These  ribs  were  bound 
together  by  nine  gussets,  G,  G,  G,  f  inch  thick,  secured  to  the  ribs  and  to  the  skin 
plates  by  angle  irons  3  inches  by  3  inches  by  f  inch.  Two  small  girders  H,  H,  were 
also  fitted  between  the  ribs  immediately  over  the  track  on  which  the  shield  moved. 

But  in  this  shield  no  horizontal  tie  girders  united  the  extremities  of  each  rib, 
nor  was  there  any  working  platform,  forming  an  integral  part  of  the  shield.  Below 
the  lower  flanges  of  the  ribs  there  was  no  framework  of  any  kind  to  impede  the 
work  of  excavating  the  face.  The  shape  of  the  shield  was  maintained  solely  by 
the  rigidity  of  the  ribs  E  and  F,  as  there  was  no  framing  to  prevent  them  spreading 
under  the  superincumbent  weight. 

The  front  hood  of  the  shield  was  supported  by  brackets,  J,  J,  of  similar  con- 
struction to,  and  in  line  with,  the  gussets  G,  G  between  the  ribs.  The  tail  of  the 

1  The  Author  is  indebted  to  Mr.  Carson  for  the  drawings  from  which  these  figures  are 
prepared. 

309 


FIG.  207.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Treniont  Street  Shield  :    Longitudinal  Section. 


TUNNEL   SHIELDS 


shield  was  unsupported 
by  brackets  ;  indeed,  its 
length,  4  feet,  was  barely 
long  enough  to  admit  of 
the  brick  arch  being  built 
under  it. 

The  ribs  rested  on 
cast  steel  shoes  of  a 
special  pattern  (for  de- 
tails see  Fig.  209).  Each 
shoe  consisted  of  two 
parts,  an  upper  casting 
bolted  to  the  rib,  and 
having  a  hemispherical 
base,  and  a  lower  one,  or 
sliding  plate,  into  a  cavity 
of  which  the  upper  one 
fitted.  The  lower  plate 
moved  over  the  track 
girders  as  already  de- 
scribed, and  the  knuckle 
joint  made  by  the  two 
parts  of  the  shoe  facili- 
tated the  movement  of 
the  shield  about  either 
end  of  the  ribs  when  a 
change  of  direction  was 
necessary. 

The  rams  for  driving 
the  shield  were  ten  in 
number,  eight  of  them 
placed  between  the  gus- 
sets G,  G,  and  one  over 
each  of  the  girders  H ,  H. 
They  were  6  inches  in 
diameter,  and  could 
work  up  to  a  maximum 
pressure  of  3,000  pounds 
per  square  inch,  which 
amount  was,  however, 
seldom  required,  1,200 
pounds  per  square  inch, 
which  was  equivalent  to 
a  total  thrust  of  over 
500  tons,  being  usually 
sufficient.  On  the  front 
rib  E  the  rams  bore  on 
castings  bolted  to  the 
web,  and  holes  for  them 


310 


THE    SHIELD    IN    MASONRY    TUNNELS 


T 


were  cut  to  correspond  in  rib  F,  the  web  of  which  was  strengthened  by  extra  plates 
round  the  holes. 

The  most  important  improvement  made  by  Mr.  Carson  consists,  however,  in 
the  introduction  of  cast-iron  bars,  2£  inches  in  diameter,  into  the  masonry  of 
the  tunnel  arch,  disposed  so  as  to  receive  each  one  the  thrust  of  one  of  the  shield 
rams.  This  idea,  Mr.  Carson  states,  was  suggested  to  him  in  the  first  place  by 
Mr.  W.  L.  Aims,  the  resident  engineer  of  the  East  River  Gas  Tunnel  in  New  York. 

These  bars  were  built  in  as  each  length  of  the  tunnel  was  constructed,  the 
ends  being  loosely  secured  in  line  with  the  preceding  ones  by  iron  sleeves  about 
3  inches  long. 

This  ingenious  arrangement  at  once  avoided  the  difficulties  met  with  in  pushing 
against  the  centres  supporting  the  masonry,  and  on  the  other  hand  in  using  the 
newly  constructed  arch  as  an  abutment. 

In  the  former  method,  employed  with  the  Chagnaud  and  Champigneul  shields, 
the  pressure  of  the  shield  always,  sooner  or  later,  produced  distortion  in  the  centres 
and  generally  rupture  in  the 
green  masonry  of  the  arch  ; 
in  the  latter,  the  use  of  the 
masonry  itself  as  a  fulcrum 
proved  entirely  destructive 
of  the  strength  of  the  mate- 
rial. 

Mr.  Carson's  arrangement 
by  avoiding  both  these  dis- 
advantages is  a  distinct  step 
in  advance  in  masonry  work 
under  shield,  nor  does  it  ap- 
pear likely  that  the  presence 

of  the  cast-iron   bars    in  the  I   | Jj I-L 

brick  arch  will,  by  destroying 
in  some  measure  its  homo- 
geneity, weaken  it  as  a  struc- 
ture. 

The  rate  of  progress  was  9  feet  per  day  of  twenty-four  hours  under  normal 
conditions. 

Behind  the  shield,  the  masonry  arch  was  carried  on  light  wooden  centres  ;  the 
removal  of  the  material  excavated  was  provided  for  by  a  trench  (see  Fig.  206) 
sunk  below  the  floor  level  of  the  shield  excavation,  and  the  materials  for  the  masons 
were  brought  forward  on  a  track  supported  on  timbers  slung  from  the  centres. 

The  space  left  by  the  skin  of  the  shield  was  filled  in  by  cement  grout  pumped 
through  pipes  left  in  the  brick  arch. 

About  450  feet  of  the  total  travel  of  the  shield  was  in  ground  composed  of 
closely  compacted  clay  and  sand,  unfavourable  to  rapid  progress,  and  sometimes 
boulders  as  large  as  4  feet  across  were  met  with.  The  remaining  100  feet  traversed 
by  the  shield  was  in  loose  sand  and  gravel,  and  in  this  portion  the  cutting  edge  of 
the  shield  was  kept  hard  up  against  the  working  face,  instead  of,  as  in  the  clayey 
portion,  removing  a  space  in  front  of  the  cutting  edge,  into  which  the  shield  could 
be  pushed  with  small  expenditure  of  power.  The  rate  of  progress  of  the  two 
systems  did  not,  however,  vary  materially. 


r- 

=1 

' 

O           O;  _ 
~~T" 

1 

\ 

V 

,-; 

O            O 

;0            0 

1 

i 


FIG.  209.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Tremont  Street  Shield  :  Cast  Steel  Shoes. 


TUNNEL    SHIELDS 


MEAN     H16H      WA  TCK 


The  period  occupied  in  driving  550  feet  of  tunnel  was  ninety-two  days,  so 
that  the  average  rate  of  progress  was  about  5*  1  feet  per  day.  When  the  shield  was 
actually  working,  however,  the  rate  of  progress  was,  as  stated  above,  nearly  9  feet 
per  day. 

The  Tunnel  under   Boston  Harbour,  or  East  Boston  Tunnel 

This  tunnel  connects  the  underground  railway  of  Boston,  of  which  the  Tremont 
Street  tunnel  just  described  forms  part,  with  the  town  on  the  other  side  of  the 
harbour  known  as  East  Boston.  The  width  of  the  harbour  where  the  tunnel  crosses 

it  is  about  2,000  feet,  but,  owing 
to  the  crossing  being  an  oblique 
one,  the  length  of  tunnel  under 
the  waterway  is  about  2, 400  feet, 
and  if  the  docks  on  either  side 
of  the  harbour  be  taken  into 
account,  the  total  length  con- 
structed under  navigable  water  is 
about  3,500  feet  (see  Fig.  203). 

The  tunnel  is  23  feet  4  inches 
wide  at  springing,  and  20  feet  6 
inches  from  invert  to  crown,  or 
somewhat  larger  than  the  Tre- 
mont Street  Tunnel,  which  in 
section  it  resembled.  It  is  built 
entirely  in  mass  concrete,  and  is, 
up  to  the  present,  much  the  most 
important  work  carried  out  in 
that  material  by  means  of  a 

v^*^^™H*T^-^*x-yw»!C3  shield.     It  was  not,  however,  the 

*«*  t™n«l  »  constructed.  In 
1897,  in  connexion  with  some 
drainage  works  near  Paris,  a 
small  circular  tunnel,  6  feet  8 
inches  in  diameter,  was  driven 
under  the  River  Oise 1  near  its 
junction  with  the  Seine,  which  is 
of  concrete,  its  method  of  con- 
struction being  much  the  same  as 
that  now  to  be  described,  save 
that  the  shield  used  followed 
English  models,  and  was  circular. 
The  maximum  depth  (see 
Fig.  210)  of  the  excavation  below 
high  water  mark  was  about  90  feet,  and  the  cover  over  the  crown  beneath  the 
harbour  bed  about  18  feet  at  the  least.  The  material  met  with  was,  however,  for 
the  most  part  strong  blue  clay  and  boulder  clay,  and  sometimes  a  sandy  and  silty 
clay  permeable  by  water.  Large  boulders  were  occasionally  met  with  and  some 
strata  of  coarse  sand. 

1  See  page  290. 
312 


•Sca/e 

FIG.  210.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
Section  of  Tunnel  under  the  Harbour. 


THE    SHIELD    IN    MASONRY    TUNNELS 

Probably  for  the  greater  part  of  its  length  the  tunnel  could  have  been  driven 
without  compressed  air,  or  at  any  rate  with  only  a  few  pounds  pressure,  but  the 
risk  involved  in  working  without  the  security  given  by  the  provision  of  compressed 
air,  under  a  wide  and  deep  waterway,  and  the  assistance  which  a  pressure  of  even 
20  pounds  per  square  inch  gives  in  holding  up  the  area  of  the  excavation,  when 
working  at  such  a  depth,  made  the  installation  of  an  air-compressing  plant  almost 
obligatory. 

The  usual  pressure  under  the  river  was  from  18  to  20  pounds  per  square  inch, 
and  only  occasionally  was  as  high  as  25  pounds.  The  amount  of  free  air  delivered 
per  man  per  hour  was  only  on  the  average  about  1,200  cubic  feet,  a  quantity  con- 
siderably below  that  usually  considered  necessary,  but  the  engineer  of  the  work 


FIG.  211.     UNDEKGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Shield  :  Longitudinal  Section,  and  detail  at  S  T,  Fig.  212. 

states  that  few  cases  of  compressed  air  sickness  were  brought  to  his  notice.  The  air 
was  delivered  behind  and  also  in  the  top  of  the  shield,  and  in  each  advance  heading 
for  the  side  walls.  The  temperature  varied  from  80°  to  90°  Fahrenheit,  and  it 
was  noted  that  the  heat  of  the  tunnel  was  increased  by  chemical  action  in  the 
concrete,  which,  two  days  after  setting,  had  an  interior  heat  40°  above  that  of 
the  air  in  the  tunnel. 

The  amount  of  carbonic  acid  in  the  air  of  the  tunnel  averaged  during  a  con- 
siderable portion  of  the  time  nearly  3  parts  in  1000,  a  much  higher  percentage  than  is 
advisable,  even  with  the  comparatively  low  pressure  employed.  But  on  occasion 
the  percentage  rose  to  6' 9  parts  in  1,000  or  nearly  '7  per  cent.,  an  entirely  dangerous 
condition  for  work. 

313 


TUNNEL    SHIELDS 


The  hours  worked  were  also  in  excess  of  those  usually  accepted  as  safe.     The 


I 


o 


o  15 
^  cc 
P 

§     I 

S  I 

ll 

t>     o 


day  was  divided  into  two  eleven-hour  shifts  from  7  a.ni.  to  6  p.m.,  and  from  7  p.m. 
to  6  a.m.,  with  a  break  of  one  hour  at  noon  and  midnight  respectively. 

3M 


THE    SHIELD    IN    MASONRY    TUNNELS 

No  safety  screens  x  were  provided  in  the  tunnel,  but  an  emergency  lock  (see 
Fig.  218)  was  fixed  in  the  upper  part  of  the  bulkhead,  above  the  working  air  locks. 

Two  air  compressors  were  provided  for  the  supply  of  air  to  the  tunnel  of  the 
Ingersoll-Sergeant  type,  with  a  combined  capacity  of  1,560  cubic  feet  of  free  air 
per  minute  at  18  pounds  pressure.  The  compressor  for  machinery  worked  at  125 
pounds  pressure  per  square  inch. 

The  conditions  of  the  specification  referring  to  compressed  air  work  were  as 
under  : — 

Special  care  and  such  appliances  as  are  requisite  shall  be  used  to  insure  the  proper  venti- 
lation of  tunnel  and  similar  work.  The  amount  of  carbonic  acid  gas  present  at  any  time  at 
the  working  faces  shall  not  be  allowed  to  exceed  one  part  to  1,000  parts  of  air.  The  contractor 
shall  amply  provide  means  and  appliances  for  the  safety  of  the  work  and  for  the  safety  and 
health  of  the  men  employed  in  it,  which  shall  include  :  electric  lighting,  telephonic  communi- 
cation between  all  parts  of  the  work :  suitable  resting  places  for  the  men,  and  a  drying  place  for 
clothes,  a  compressed-air  chamber  fitted  with  bunks  if  the  air  pressure  exceeds  20  pounds  per 
square  inch  above  the  normal  :  an  emergency  air  lock  (in  addition  to  the  ordinary  working 
air  locks)  in  each  airtight  bulkhead  for  the  tunnel,  with  access  thereto  from  the  ordinary  work- 
ing levels  :  arrangements  such  that  it  shall  not  be  necessary  for  men  to  climb  any  stairs  imme- 
diately after  getting  out  (of  the  air-chamber)  ;  keeping  every  portion  of  the  work  in  a  thoroughly 
sanitary  condition  ;  and  dry  earth  closets  and  other  necessary  conveniences  for  the  men. 


JtcUfFLa^  of  VfifierI'Loor  JJaJLF^Lcui.  oTLonrer Floor. 

FIG.  213.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Shield  :    Sectional  Plan   on  line  0  P,   Fig.    212. 

The  shield  employed  is  shown  in  Figs.  211,  212,  213,  214  and  215.2  Two 
shields  were  constructed,  that  shown  being  the  second  one  used,  the  only  differences 
between  the  two  being  in  some  variations  in  the  proportions  of  the  joists  and  rollers 
on  which  the  shield  travelled,  and  in  an  alteration  made  at  the  crown  of  the  rear 
girder,  whereby  the  access  to  the  new  concrete  arch  for  keying  it  up  was  improved. 

The  general  arrangement  of  the  framework  resembles  the  French  models 
much  more  closely  than  did  the  Tremont  Street  Shield.  The  two  main 

1  See  pages  206,  243  and  261. 

2  The  Author  is  indebted  to  Mr.  Carson  for  the  drawing  from   which  these  plates  are 
prepared. 

315 


TUNNEL    SHIELDS 


ribs,  a,  b,  2  feet  deep,  with  their  horizontal  ties,  c,  d,  and  their  vertical  frames,  e,  e, 
f,  /,  are  so  rigidly  bound  together,  not  only  by  the  usual  gussets,  g,  g,  but  by  other- 
plates  connecting  the  vertical  and  horizontal 
frames,  that  they  practically  form  one  fram- 
ing. 

The  plates  connecting  the  tie  girders, 
c  and  d,  form  the  lower  platform  of  the  shield, 
and  the  upper  platform,  h,  is  carried  on 
double  channel  bars,  j,  j,  which  brace  the 
two  ribs. 

Great  strength  is  obtained  by  thus 
making  the  body  of  the  shield  one  solid 
frame  ;  on  the  other  hand  the  working  area 
is  seriously  limited  by  the  number  of  brac- 
ings, and,  as  will  be  seen  from  Fig.  212,  the 
largest  apertures  in  the  centre  of  the  shields 
are  8  feet  2  inches  wide,  and  4  feet  1  \  inches 
and  3  feet  9  inches  high  respectively.  These 
formed  the  working  area  of  the  shield,  the 
portions  of  the  framing  under  the  haunches 
being  filled  with  the  hydraulic  machinery. 

The  ribs  and  frames  rested  at  either 
end  on  longitudinal  girders,  formed  of  five 


Scale 


FIG.  214.    UNDERGROUND  RAILWAY,  BOSTON, 

U.S.A. 

The  Harbour  Shield  :    Section  on  line  E  F, 
Fig.  212. 


key 


rolled  joists  9  inches  deep  (6  inches  deep  in 
the  first  shield),   having  a  top  flange  \  inch 

thick  and  a  bottom  flange  1  inch  thick.  The  length  of  these  girders  was  6  feet 
6  inches,  which  amounted  to  50  per  cent,  of  the  length  of  the  skin  at  the  crown,  a 
proportion  much  more  reasonable  than  in  most  of  the  roof  shields  noticed  previously. 

In  the  first  of  the  two  shields  the  rear  rib, 
b,  was  made  2  feet  deep  throughout ;  in  the 
second  it  was  cut  away  at  the  crown  (see 
Fig.  212)  to  facilitate  the  keying  up  of  the 
concrete  arch. 

Sixteen  hydraulic  jacks,  worked  by 
pumps  with  a  capacity  of  4,000  pounds  per 
square  inch  fixed  in  the  shield,  were  fitted  in 
the  shield  between  the  ribs,  and  bore,  not  on 
the  centres  supporting  the  concrete  arch, 
but  on  sixteen  lines  of  cast-iron  bars,  each 
3£  inches  in  diameter  and  30  inches  long, 
imbedded  in  the  concrete  arch,  each  bar 
being  loosely  connected  to  the  preceding  one 
by  a  sleeve  of  cast  iron  (see  Fig.  219). 
Generally  the  shield  moved  with  a  total 
pressure  of  20  to  30  tons. 

The  shield  travelled  on  rollers  (see  Figs. 
215  and  218)  framed  together  much  as  are 

similar  rollers  in  the  expansion  bearings  of  a  girder  bridge,  and  these  again  moved 
on  plates  flanged  to  assist  in  guiding  the  shield  fixed  on  the  top  of  the  concrete 

316 


FIG.  215.  UNDERGROUND  RAILWAY,  BOSTON, 

U.S.A. 
The  Harbour  Shield  :  Details  of  Rollers. 


3*w?^~ 


CROSS    SECTION    SHOW/ING    SIDE    DRIFTS 


CROSS    SECTION    SHOWING    WAU.    IN   DRIFTS 

FIG.  216.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Tunnel  :    Method  of  working. 


317 


TUNNEL    SHIELDS 

sidewalls  built  in  advance  of  the  shield.  As  the  shield  moved  forward,  the  rearmost 
roller  was  removed,  and  fixed  in  the  front  again,  eighteen  rollers  being  used  on 
each  path. 

The  weight  of  the  shield,  exclusive  of  the  hydraulic  jacks  and  pumping  gear, 
was  about  52  tons. 

The  method  of  working  followed  closely  that  adopted  in  the  Tremont  Street 
Tunnel.  A  shaft  was  sunk  at  the  East  Boston  end  of  the  section  to  be  constructed 
under  shield  to  the  required  depth  of  42  feet,  and  in  it  were  at  once  constructed  the 
permanent  invert  and  side  walls,  which  latter  were  carried  up  to  within  1  foot 
4  inches  of  the  springing  line  of  the  arch,  that  being  the  level  of  the  tracks  or  plates 
on  which  the  shield  was  to  run.  From  the  shaft  two  headings,  each  8  feet  square, 
(see  Fig.  216)  were  driven,  one  for  each  side  wall  of  the  tunnel,  in  advance  of  the 
shield.  The  timbering  of  the  headings  consisted  of  8  inch  by  8  inch  spruce  fir 
headtrees,  with  similar  sidetrees  on  the  inside  side  of  the  headings,  but  having 
on  the  outer  side  the  headtrees  supported  by  a  longitudinal  timber  6  inches  by 
8  inches,  supported  on  props  8  inches  by  8  inches,  and  2  feet  6  inches  apart,  centre 
to  centre,  which  again  rested  on  a  foot  block,  or  rather  longitudinal  timber,  8  inches 
square. 

This  framing  was  further  strengthened  by  struts  between  the  inner  and  outer 
sidetrees,  placed  just  high  enough  to  clear  the  concrete  invert.  The  outside  face 
was  close-poled  between  the  sidetrees  with  2-inch  boards.  It  was  found  that  on 
exposure  to  the  air  the  clay  swelled  in  a  few  hours,  often  sufficiently  to  crush  the 
timbering. 

The  material  excavated  was  removed  by  hand  barrows.  When  the  headings 
had  been  driven  about  40  or  50  feet,  the  bottoms  (see  Fig.  216)  were  excavated,  and 
the  foundations  of  the  walls  and  a  small  portion  at  each  end  of  the  invert  was  put 
in  lengths  of  from  16  to  20  feet.  When  the  concrete  had  set  the  weight  of  the  roof 
was  transferred  from  the  outside  sidetrees,  by  fixing  under  it,  3  feet  from  its  outer 
end,  a  longitudinal  timber  6  inches  by  8  inches,  which  was  supported  on  posts 
3  feet  apart,  and  wedged  in  the  concrete  of  the  invert.  When  this  was  made  secure, 
the  outside  trees  and  poling  boards  were  removed,  and  the  concrete  wall  put  in 
up  to  within  16  inches  of  the  springing  line,  the  inside  face  being  filled  against 
shuttering  fixed  to  the  posts  previously  put  in. 

The  shield  having  been  erected  on  the  side  walls  built  in  the  shaft,  it  was  forced 
into  the  face  of  earth,  the  bulkhead  having  been  removed,  and  as  it  advanced  in 
lengths  of  2  feet  6  inches  at  a  time,  the  concrete  arch  was  constructed  behind  it 
under  the  tail  of  the  shield  (see  Fig.  218),  the  cast-iron  bars  for  receiving  the  thrust 
of  the  shield  rams  being  built  into  it.  These  bars  remained,  of  course,  in  the 
arch,  and,  in  addition,  a  crown  bar  of  twisted  steel  was  left  in  each  length, 
and  temporary  tie  bars  were  also  placed  on  each  side  by  wThich  the  green  concrete 
was  anchored  to  the  centres.  The  new  masonry  was  supported  in  centres  made 
of  steel  10-inch  channel  bars  and  spaced  2  feet  6  inches  apart.  Logging  4  inches 
thick  was  placed  on  the  ribs  to  hold  up  the  concrete.  Timber  ribs,  similar  to  the 
grouting  ribs  used  in  English  shields  in  cast-iron  lined  tunnels,  were  used  to  hold 
the  fresh  concrete  in  its  place,  the  thrust  of  the  ribs  being  taken,  however,  en- 
tirely by  the  cast-iron  rods.  The  keying  up  of  the  arch  presented  some  difficulty. 
In  the  first  shield  built,  the  back  rib,  being  2  feet  deep  at  the  crown,  made  the  key 
of  the  arch  almost  inaccessible,  but  the  difficulty  was  got  over  by  using  the  two 
uppermost  apertures  in  the  web  of  the  rib,  which  was  constructed  to  admit  of 


PtMSC 
CROSS     SECTION     SHOWING     SHIELD 


PHASE 
CROSS     SECTION    SHOWING    CENTRE 

FIG.  217.     UNDERGROUND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Tunnel  :    Method  of  Working. 


319 


LONGITUDINAL    SECTION    AT    SHIELD 


CROSS    SECTION    SHOWING   AIR    LOCKS 

FIG.  218.     UNDEBGKOUND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Tunnel :    Method  of  Working. 


320 


THE    SHIELD    IN    MASONRY    TUNNELS 

eighteen  rams  being  used,  only  sixteen,  however,  being  actually  fitted.  Curved 
sheet-iron  troughs  were  fitted  between  these  openings  and  the  top  of  the  arch,  and 
concrete  fed  into  these  troughs  from  the  front  of  the  rib  was  pushed  by  properly- 
shaped  rammers  into  the  crown  of  the  arch  to  key  it  up.  This  difficulty  was  avoided 
in  the  second  shield  by  making  the  crown  of  the  rear  rib,  b  (Fig.  212),  very  shallow, 
and  so  permitting  access  to  the  key. 

The  space  left  vacant  over  each  new  length  of  the  arch  ring  by  the  advance- 
ment of  the  tail  of  the  shield  was  filled  with  grout  composed  of  two  or  three  parts 
of  sand  to  one  of  cement,  which  was  forced  by  air  pressure  through  a  vertical  pipe 
built  into  the  arch  for  the  purpose.  In  England  neat  lias  lime  is  always  used 
for  grouting  up  cavities  of  this  character,  mainly  because  it  is  less  likely  to  set 
and  choke  the  pipe  during  the  process  of  filling  than  is  cement,  if  used  neat,  and 
because  the  proper  mixing  of  cement  with  a  due  proportion  of  sand,  which  would 
retard  the  setting,  would  occupy  too  much  time  and  also  space. 


SECTION    THROUGH    HYDRAULIC    JACKS 
AND   PUSH    BARS 


6/</«  !**"• 
OCTAIL  Of    PUSH 


FIG.  219.     UNDEKGBOTJND  RAILWAY,  BOSTON,  U.S.A. 
The  Harbour  Shield  :    Details  of  Rams  and  Push  Bars. 

A  sufficient  number  of  centres  to  allow  of  each  one  remaining  in  position  for 
thirty  days  were  provided. 

The  character  of  the  concrete,  as  provided  in  the  specification,  was  that  to 
each  123  pounds  of  dry  Portland  cement  there  should  be  2|  cubic  feet  of  sand 
and  4  cubic  feet  of  gravel.  Roughly  this  meant  about  6  to  1  cement  concrete. 
Crushed  stone  in  place  of  gravel,  and  fine  crushed  stone  in  place  of  sand,  were  also 
used. 

There  does  not  appear  to  have  been  any  marked  shrinkage  1  in  the  concrete 
roof,  and  what  little  there  was  was  of  little  importance,  due  to  the  fact  that  the 
tunnel  was  for  the  greater  part  of  its  length  surrounded  by  solid  clay.  But  the 
successful  use  of  cement  concrete  in  the  favourable  conditions  here  met  with  does 
not  establish  any  ground  for  preferring  it  to  cast  iron  as  a  tunnel  lining  in  material 
where  any  risk  of  a  large  inrush  of  water  may  be  possible,  nor  does  it  affect  the 
fact  that,  at  present  prices,  the  employment  of  a  cast-iron  lining  would,  in  England, 
by  reason  of  the  reduction  in  the  amount  of  excavation  made  by  its  use,  work  out 
at  about  the  same  price  per  yard  run  of  tunnel,  and  would  at  the  same  time 
allow  the  daily  rate  of  progress  to  be  increased.  In  the  case  of  a  tunnel  in  water- 
bearing material,  the  rapidity  with  which  a  cast-iron  lining  can  be  erected  (and 

1  Observations  were  taken  on  experimental  beams  of  concrete  8'9  feet  long  and  8  inches 
square.  It  was  found  that  the  shrinkage  in  twelve  weeks  amounted  to  0'028  per  cent.  If 
the  concrete  were  immersed  in  water,  the  shrinkage  was  about  0'009  per  cent. 

321  Y 


322 


THE    SHIELD    IN    MASONRY    TUNNELS 


attain  its  full  strength  at 
once),  and  the  ease  with 
which  it  can,  by  caulking,  be 
made  practically  watertight, 
make  it  always  the  preferable 
material.  In  the  Boston 
Tunnel  Mr.  Carson  estimates 
that  at  the  end  of  thirty  days 
the  arch  was  of  only  one-half 
its  permanent  strength. 

The  excavation  of  the 
dumpling  between  the  side 
walls  was  done  at  the  same 
time  as  the  arch  was  being 
built  (see  Fig.  218).  In 
general  the  side  walls  were 
built  about  100  feet  in  ad- 
vance of  the  shield,  and  the 
alignment  of  the  tunnel  was 
kept  by  two  parallel  lines 
being  run  along  the  headings 
for  these  walls.  The  invert 
of  the  tunnel  was  constructed 
later,  usually  in  lengths  of 
about  10  feet. 

When  the  shield  had 
advanced  some  200  feet,  al- 
most entirely  in  clay  with 
silt  and  coarse  sand  at  the 
crown,  but  little  water  being 
met  with,  a  halt  was  made 
to  allow  of  air-locks  and 
bulkhead  being  put  in  (see 
Figs.  218,  220  and  221). 

Three  locks  were  pro- 
vided, the  two  lower  ones  for 
working  purposes,  the  upper 
one  as  an  emergency  lock. 
These  locks  were  27  feet  long 
and  6  feet  in  diameter,  and 
were  built  in  a  brick  bulk- 
head which  differed  from 
those  previously  described  in 
this  book,  in  being  simply  a 
dome  -  shaped  mass  3  feet 
thick  having  a  versed  sine 
of  6  feet,  and  strengthened 
with  iron  hoops.  This  dome 
was  keyed  into  the  concrete 
tunnel  lining. 


323 


TUNNEL    SHIELDS 

The  working  arrangements  of  the  tunnel  are  shown  in  Figs.  220,  221,  the  only 
feature  requiring  comment  being  the  provision  of  an  upper  track  for  the  conveyance 
of  concrete  for  the  arch.  This  track  was  carried  on  joists  fixed  on  the  centres, 
to  which  they  formed  struts,  and,  when  the  centres,  as  they  became  unnecessary 
were  moved  forward,  on  timber  frames. 

The  quantity  of  material  to  be  handled  amounted  to  about  22  cubic  yards 
of  excavation,  and  8  cubic  yards  of  concrete  per  yard  forward.  These  quantities 
include  the  whole  of  the  tunnel  from  invert  to  crown. 

When  work  was  in  regular  progress  under  compressed  air,  the  average  weekly 
advance  was  about  32  feet,  the  maximum  distance  travelled  in  one  week  being 
45  feet. 

The  work  was  on  the  whole  singularly  free  from  accidents,  either  to  the 
machinery  or  to  the  men  employed.  On  two  occasions,  however,  the  frames  of 
rolled  joists  which  formed  the  bases  of  the  shield  on  each  roller  path  gave  way,  owing 
to  some  irregularities  in  the  levels  of  the  side  walls.  To  make  these  up  to  the 
necessary  height  to  receive  the  plates  on  which  the  rollers  travelled,  timber  joists 
were  laid  on  them,  and  these  joists  crushed  under  the  load  and  in  consequence 
put  such  a  strain  on  the  rolled  joists  over  the  rollers  that  they  crippled  and  had  to 
be  renewed. 

The  accidents  were  unimportant,  and  merely  emphasize  the  fact  that  the 
weak  point  in  all  roof  shields  is  that  all  the  weight  over  the  whole  area  of  the  shield 
is  thrown  on  a  very  limited  bearing  surface,  the  soundness  of  the  foundations  of 
which  depend  on  conditions  outside  of  the  shield  itself. 

The  period  occupied  in  driving  the  tunnel  under  the  river  (which  work  formed 
the  first  section  of  the  East  Boston  Tunnel)  was  from  January  26,  1901,  when  the 
working  shaft  near  Sumner  Street,  East  Boston,  was  completed,  to  June  30,  1903, 
the  distance  driven  under  shield  being  4,280  feet.  This  gives  an  average  rate 
per  day  of  4'8  feet  or  about  33  feet  per  week,  a  very  satisfactory  average,  con- 
sidering that  the  time  required  for  the  installation  of  the  air-locks  and  fittings,  and 
all  interruptions  such  as  those  necessitated  by  repairs  to  the  shield,  track  shifting^ 
and  the  like  are  included  in  the  period  given  above. 

The  Metropolitan  Railway  of  Paris 

Schemes  for  the  construction  of  an  underground  railway  in  Paris,  on  similar 
lines  to  that  constructed  in  London,  have  during  the  last  forty  years  been  numerous, 
and  have  been  the  subject  of  many  parliamentary  inquiries  and  discussions.  It 
was  not,  however,  until  1898,  when  the  near  approach  of  the  great  international 
exhibition  of  1900  made  improved  transit  facilities  in  the  city  a  matter  of  urgency, 
that  a  general  scheme  of  urban  lines  under  powers  conferred  by  the  State  was 
approved  by  the  Conseil  Municipal  of  Paris,  and  a  commencement  made  with  the 
most  important  part  of  it,  namely  the  line  connecting  the  Porte  de  Vincennes  in  the 
east  with  the  Porte  de  Maillot  in  the  Avenue  du  Bois  de  Boulogne  in  the  west  of 
Paris.  This  line  (see  map,  Fig.  222)  was  opened  to  traffic  in  July,  1900,  and  in 
the  same  year  the  second  or  northern  portion  from  the  Arc  de  FEtoile  to  the  Place 
de  la  Nation  via  Batignolles,  Montmartre  and  Menilmontant  was  commenced,  the 
complete  circuit  being  finished  in  1903. 

The  construction  of  an  urban  line  of  such  magnitude  (the  lines  to-day  open 
to  public  traffic  are  16£  miles  in  length)  presents  innumerable  interesting  engineering 

324 


features,1  every  known  method  of  tunnelling  being  employed  to  suit  the  varied 
conditions  under  which  the  underground  works  had  to  be  carried  out,  but  only 
those  sections  of  the  line  where  shields  were  used  are  treated  of  here,  and  it  is 
somewhat  notable  that,  in  spite  of  the  success  which  had  attended  the  use  of 
shields  in  the  construction  of  masonry  tunnels  in  Paris  in  the  years  immediately 
preceding  the  commencement  of  the  work,  on  the  whole  so  little  effective  use 
was  made  of  this  method  of  tunnelling  in  an  undertaking  which  for  most  of  its 
length  involved  the  driving  of  a  double-line  tunnel  under  busy  thoroughfares 
where  cut-and-cover  work  was,  except  in  special  cases,  entirely  prohibited. 

In  the  second  or  northern  part  of  the  railway  no  shields  were  employed,  and 
in  the  part  first  constructed  from  Vincennes  to  the  Quartier  de  1'Etoile,  out  of 
eleven  sections  into  which  the  line  was  divided  they  were  not  used  at  all  in  four, 
were  tried  and  abandoned  in  three,  and  in  four  sections  only  appear  to  have  achieved 
a  certain  measure  of  success. 


FIG.  222.     PLAN  OF  PARIS  SHOWING  POSITION  OF  TUNNELS  BUILT  UNDER  SHIELD. 

Again,  the  employment  of  the  shield  was,  in  the  sections  where  it  was  adopted, 
limited  to  one  type  of  tunnel  only,  a  masonry  tunnel  for  two  lines  of  rails,  of  the 
section  shown  in  Fig.  227,  the  wider  station  tunnels  and  the  single  line  ones  con- 
structed in  connexion  with  some  of  these  being  all  built  in  timbered  lengths,  and 
of  course  this  was  the  case  with  the  bell-mouthed  approaches  to  stations,  a  form  of 
construction  which  was  largely  used. 

It  resulted  from  this  limitation  that  of  the  total  length  of  8|  miles  of  the 
Vincennes-Etoile  line  and  branches  only  a  little  more  than  1£  miles  of  tunnel  were 
constructed  under  shield,3  and  apparently  the  results  were  not  too  satisfactory,  as 
although  the  employment  of  a  shield  for  the  construction  of  the  double  line  tunnels 

1  The  only  account  of  the  work  up  to  the  present  which  gives  a  description  of  the  whole 
of  the  undertaking  is  M.  Jules  Hervieu's  Chemin  de  Fer  Metropolitan  du  Municipal  de  Paris, 
Paris,  1903,  which  is  the  official  history  of  the  railway. 

2  See  Philippe's  Le  Bouclier,  in  which  considerable  space  is  given  to  a  description  of  the 
somewhat  unsatisfactory  work  on  this  line. 

325 


TUNNEL    SHIELDS 

was  originally  made  a  condition  of  the  contracts  for  the  various  sections  of  the 
first  or  Vincennes-Etoile  line,  their  use  in  the  second  or  northern  line  was  permissive 
only,  and  as  a  fact  shields  were  not,  as  stated  above,  used  in  that  part  of  the  railway 
at  all. 

A  feature  of  all  the  shield  work  was  the  return,  save  in  one  section,  the  fourth 
(in  which  however  the  shield  method  of  working  was  hardly  tried,  so  complete 
was  the  failure  of  the  machine  used),  to  the  system  of  the  Collecteur  de  Clichy,  in 
which  the  first  operation  was  the  driving  of  the  shield  for  the  construction  of  the 
arch,  the  building  of  the  side  walls  being  left  until  the  first  stage  was  completed. 

This  system  of  working,  whatever  other  inverts  it  possesses,  has  the  grave 
demerit  in  the  case  of  tunnelling  through  a  material  of  varying  character,  but  nearly 
always  of  uneven  stiffness,  such  as  made  ground  broken  by  old  foundations  resting 
on  gravel  and  sand,  of  requiring  that  the  shield  advances  on  platforms  slight  in 
character,  built  in  short  lengths,  and  yielding,  as  the  weight  of  the  shield  comes 
upon  them,  more  or  less  as  the  material  beneath  has  more  or  less  consistency. 

It  is  probably  to  this  method  of  working,  and  also  to  the  fact  that  many  of 
the  shields  employed  were  constructed  with  a  length  of  base  remarkable  even 
among  roof  shields  for  its  disproportion  to  the  length  over  all  of  the  roof,  that 
much  of  the  ill  success  which  attended  the  use  of  shields  on  this  railway  was  due. 

As  an  example  of  the  curiously  disproportionate  planning  of  the  shields  it 
may  be  mentioned  that  one  of  them  had  a  base  only  two-ninths  of  its  total  length, 
and  two-fifths  of  its  height,  while  the  tail  was  twice  the  length  of  the  base. 
Naturally,  when  such  an  engine  advanced  on  an  unstable  bed,  its  structural 
instability  and  tendency  to  rock  was  assisted  by  the  nature  of  its  support,  and 
in  consequence  the  guiding  of  it  became  an  impossibility. 

Of  the  various  shields  employed  two  types  only  met  with  some  degree  of 
success  ;  of  the  first,  known  as  the  Champigneul  shield,  two  were  employed  on  the 
first,  one  on  the  eighth,  and  one  was  installed  but  not  used  on  the  eleventh  section  ; 
of  the  second  type,  the  Lamarre  shield,  two  were  used,  one  on  the  sixth  and  one  on 
the  seventh  section. 

The  smallness  of  the  part  played  by  shields  in  the  construction  of  the  Vincennes- 
Etoile  section  of  the  railway  is  clearly  shown  in  the  following  table. 

TOTAL  LENGTH  OF  LINE,  INCLUDING  BRANCHES,  TO  PORTE  MAILLOT,  AND  TO  THE  TROCADERO 

=  8|  MILES  ABOUT. 


Section. 

Length  of 
Section. 
Feet. 

Length  built 
under  Shield. 
Feet. 

I. 

2. 
3. 

Porte  de  Vincennes  to  the  Rue  de  Reuilly    
Rue  de  Reuilly  to  the  Rue  Lacuee      
Rue  Lacuee  to  St.  Paul 

5,885 
4,380 
3  734 

3,854 
132 
1  614 

4. 

St.  Paul  to  the  Chatelet        .... 

3  803 

459 

6, 

Chatelet  to  the  Tuileries        

4  351 

6. 

Tuileries  to  the  Champs  Elysees 

4  089 

295 

7. 
8. 
9. 
10. 
11. 

Champs  Elysees  to  the  Avenue  de  1'Alma      
Avenue  de  1'Alma  to  the  Porte  Maillot     
Avenue  de  Wagram  to  the  Place  Victor  Hugo    
Place  Victor  Hugo  to  the  Porte  Dauphine      
Place  de  1'Etoile  to  the  Place  du  Trocadero        

3,825 
4,593 
3,555 
2,444 
5,121 

689 
1,309 

Total  length  in  feet 

45  780 

8  352 

326 


THE    SHIELD    IN    MASONRY    TUNNELS 


Thus  a  little  over  18  per  cent,  of  the  whole  length  of  the  line  was  constructed 
under  shield,  and  even  if  the  length  of  the  stations  which  constitute  an  appreciable 
part  of  the  whole  mileage  of  an  urban  railway  be  deducted  from  the  total  length, 
the  proportion  of  the  remainder  in  which  shields  were  employed  was  only,  assuming 
the  total  length  of  the  stations  at  8,200  feet,  about  22  per  cent.1 

The  eleven  shields  constructed  for  the  railway  were  distributed  as  under  : — 

Section  1.     2  shields,  type  Chanipigneul 

2.     1  Baudet  Donon  &  Cie. 


3.  2 

4.  2 

6.  1 

7.  1 

8.  1 
11.  1 


,,  ,,  ,,     (Dieudonnat) 

Moranne.   (Weber) 
Baudet  Donon  &  Cie.   (Lamarre) 

Chanipigneul 


It  will  be  seen  that  the  shields  of  the  Chanipigneul  type,  in  spite  of  the  complete 
abandonment  of  the  one  erected  for  work  in  the  eleventh  section,  nevertheless  were 
employed  in  building  5,163  feet,  or  more  than  five-eighths  of  the  total  length  of 
8,352  feet  constructed  under  shield. 

Of  the  remainder  the  Lamarre  shield  was  employed  in  the  construction  of 
689  feet  of  tunnel,  and  the  Dieudonnat  shield  in  a  length  of  1,746  feet.  This  last, 
however,  proved  so  unsatisfactory  that  it  is  not  easy  to  understand  how  it  was 
employed  even  over  the  distance  recorded. 

Shields  of  the  ist  and  8th  Sections  of  the  Paris  Metropolitan  Railway 

The  Chanipigneul  shield 2  and  the  centreing  connected  therewith  is  illustrated 
in  Figs.  223,  224,  225,  and  226,  which,  though  the  details  of  the  work  varied  somewhat 
in  the  different  sections,  show  the  general  arrangement  of  all  the  installations 
sufficiently  well,  as  the  four  machines  built,  three  of  which  were  used,  came  from 
the  same  factory,  and  were  practically  alike,  resembling  in  general  features  the 
Chagnaud  Shield  of  the  Collecteur  de  Clichy. 

The  shield  shown  in  the  figures  is  the  one  employed  on  the  Vincennes  length 
of  the  first  section  of  the  railway. 

The  central  framing  of  the  shield  consisted  of  two  elliptical  girders,  A,  A,  6  feet 
5  inches  apart  centre  to  centre,  and  measuring  23  feet  3  inches  along  the  major  axis, 
the  height  from  the  springing  line  being  8  feet  9|  inches.  At  the  crown  they  were 
2  feet,  and  at  the  springing  line  2  feet  7 \  inches  deep,  the  webs  being  -f  inch  thick, 
and  the  angle  irons  which  formed  the  flanges  3J  by  3J  by  \  inches. 

They  were  braced  on  the  horizontal  axis  in  the  usual  manner  by  two  girders, 
B,  B,  which  in  turn  were  united  by  nine  transoms,  C,  serving  as  supports  to  the 
working  platform  D. 

Sixteen  plate  gussets,  E,  E,  between  them  completed  the  frame,  the  spacing  of 
these  being  arranged  so  that  each  pair  of  gussets  formed  the  base  of  one  of  the 
shield  rams. 

The  skin  of  the  shield  was  T70  inch  thick,  the  tail  piece  being  doubled,  or  nearly 
1|  inches  thick.  The  total  length  at  the  crown  was  23  feet  2  inches,  the  forward 
hood  measuring  8  feet  2  inches,  and  the  tail  8  feet  7  inches.  At  the  bottom  of  the 
shield  the  hood  extended  4  feet  5  inches  in  front  of  the  leading  girder  B,  and  the 

1  There  were  twenty-five  stations,  with  a  platform  length  of  from  75  to  100  metres. 

2  The  Author  is  indebted  to  M.  Legouez  for  the  drawings  of  this,  and  of  the  Lamarre  shield. 

327 


TUNNEL    SHIELDS 


tail  of  the  shield,  which  at  the  springing  level  was  nearly  the  same  length  as  at  the 

crown,  was  then  cut  back,  and 
at   the  base  only  extended  5 
the 


feet  6  inches  behind  tne  rear- 
most main  girder. 

The  base  of  this  shield, 
therefore,  is  '28  of  its  total 
length,  or  well  under  one-third. 

The  overhang  of  the  front 
hood  and  of  the  tail  were  sup- 
ported by  sixteen  brackets, 

F,  F,  similar  in  construction, 
and  aligned  with    the  bracing 
girders,    E,     E,    between    the 
main  ribs,  A,  A,  and  the  hood 
was  stiffened  by  two  channels, 

G,  6  inches  by  2|  inches  by  2^ 
inches,  rivetted  on  either  side 
of  a  plate  6  inches  by  ?  inch. 

In  case  of  necessity  the 
hood  could  be  further  ex- 
tended for  the  protection  of 
the  miners  by  sliding  poling 
bars  between  the  hood  and 
angle  irons  fixed  for  the  pur- 
pose inside  it.  When  any 
such  protection  was  actually 
required,  however,  the  miners 
usually  preferred  to  pole  from 
the  top  of  the  cutting  edge 
in  the  usual  way. 

The  shield  rested,  not  on 
rollers  moving  on  a  roller  path 
beneath  them,  but  bore,  by 
means  of  cast-iron  shoes,  H,  H, 
on  cast-iron  rails,  J,  J,  which 
were  laid  on  transverse  tim- 
bers, K,  K.  (Fig.  225.) 

The  rams  employed  on 
the  shield,  eight  in  number, 
were  similar  in  character  to 
those  of  the  Orleans  railway 
shield,  and  had  a  stroke  of  3 
feet  4  inches,  with  a  piston 
diameter  of  9J  inches,  and  the 
hydraulic  pressure  was  about 
1^  tons  per  square  inch. 

M.  Philippe  states  *  that  the  shield  was  sometimes  driven  with  a  total  pressure 

1  Le  Bouclier,  p.    141. 
328 


THE    SHIELD    IN    MASONRY    TUNNELS 

on  the  eight  rams  of  144  tons,  and  that  the  total  pressure  never  exceeded  500  tons  ; 
this  gives  a  maximum  pressure  on  the  skin  of  the  shield — assuming  that  the  pressure 


is  uniform  on  the  sides  and  top,  which  is  hardly  likely — of  about  1,500  pounds  per 
square  foot. 

The  position  of  the  machinery  for  driving  the  shield  is  shown  in  outline  in 

329 


TUNNEL    SHIELDS 


Fig.  225.  This  was  contained  in  a  housing  L,  built  in  the  centre  of  the  shield,  the 
pump  M  ,  a  three-throw  one,  being  fixed  on  the  main  platform  of  the  shield,  and 
the  electric  motor  N  for  driving  it,  and  the  valves  P  for  working  the  rams,  being 
disposed  on  a  second  stage  above. 

The  push  of  the  shield  occupied  about  twenty  minutes  in  favourable  circum- 
stances. 

The  shield  used  on  the  Vincennes  length  of  the  first  section  had  slung  from 
its  front  hood  a  working  platform  R,  to  enable  the  miners  to  get  at  the  upper  part  of 
the  face,  and  from  the  tail  of  the  shield  was  also  hung  a  removable  platform  for  the 
masons  working  on  the  upper  part  of  the  arch.  These  arrangements  were  altered 
somewhat  in  the  shield  used  on  the  eighth  section,  when  the  miners  at  the  face  did 
not  work  at  two  levels  under  the  hood  of  the  shield,  but  got  the  excavation  by  cutting 
gullets  in  the  face,  leaving  between  them  walls  of  material  which  were  cut  away 


.J 


FIG.  225.     METROPOLITAN  RAILWAY,  PARIS. 
The  Champigneul  Shield  :    Cross  Section. 

by  the  cutting  edge  of  the  shield  in  its  advance.  The  platform  for  the  masons 
also,  instead  of  being  suspended  from  the  roof  as  shown  in  the  figure  of  the  Vincennes 
shield,  was  fixed  on  a  light  frame  attached  to  the  shield  itself,  and  rolling  on  the 
siding  rails  behind  it,  an  arrangement  saving  some  labour  as  the  platform  thus 
advanced  with  the  shield. 

The  method  of  working  adopted  with  the  Vincennes  shield,  and  with  that 
employed  on  the  eighth  section  of  the  railway,  differed  in  one  respect  from  the 
arrangements  previously  employed  with  similar  machines  in  earlier  works.  While 
the  shield  of  the  Collecteur  de  Clichy  (extra-muros) 1  advanced  without  any  previous 
heading  having  been  driven,  its  base  resting  on  the  tracks  laid  on  the  undisturbed 
material  beneath— and  on  the  other  hand,  the  shield  of  the  Orleans  Railway  exten- 
sion 2  moved  on  brick  footwalls  previously  built  in  headings  driven  in  advance  for 


See  Fig.  185. 


2  See  Fig.  200. 


330 


THE    SHIELD    IN    MASONRY    TUNNELS 

the  purpose,  the  central  dumpling  being  the  last  part  of  the  excavation  to  be 
removed — there  was  in  the  case  of  the  shield  now  under  consideration  a  central 
heading  driven  in  advance  of  and  below  the  shield,  which  was  otherwise 
operated,  as  was  the  Clichy  shield,  moving  on  tracks  laid  direct  on  the  soil 
beneath. 

This  heading  became,  as  the  advance  of  the  shield  removed  the  ground  above 
it,  a  trench  or  cutting  accommodating  a  line  of  rails  for  a  service  of  skips,  into 
which  the  spoil  from  the  front  of  the  shield  could  be  shovelled  directly,  thus  facili- 
tating the  work,  not  merely  by  the  easy  removal  of  the  spoil,  but  by  the  greater 
freedom  of  movement  the  arrangement  afforded  on  the  working  platform  of  the 
shield,  and  by  the  upper  line  of  rails  being  always  available  for  the  bringing  up  of 
material  for  the  masons,  etc. 

The  use  of  this  heading  is  open  to  the  objection  that  in  its  construction  the 
soil  on  each  side  of  it  on  which  the  shield  moves  may  be  disturbed,  and  so  produce 
settlements  when  the  weight  of  the  shield  comes  on  it. 

The  cost  of  the  excavation  of  the  heading  is  of  course  greater  than  if  the  same 
material  were  removed  behind  the  shield,  but  from  the  fact  that  although  with  the 
Reuilly  shield  on  the  second  part  of  the  first  section  ot  the  line,  a  trench  of  similar 
dimensions  to  the  heading  shown  in  the  drawings  was  made  behind  the  shield  to 
accommodate  a  line  of  rails,  the  heading  was  adopted  with  the  two  other  sections 
which  were  commenced  later,  it  would  seem  that  the  extra  cost  of  the  heading 
was  repaid  by  the  economy  effected  by  its  means  in  working  the  shield. 

With  the  Vincennes  shield  this  heading,  S,  S,  Figs.  223,  224  and  225,  was 
generally  kept  about  250  to  300  feet  in  advance  of  the  shield,  its  section  being  in 
section  nearly  a  square,  with  sides  6  feet  6  inches  long. 

The  material  passed  through  was  not  water-bearing,  but  consisting  as  it  did 
of  made  ground  and  soft  sand  for  the  most  part,  with  some  marl,  it  was 
necessary  to  timber  the  heading  ;  the  frames,  T,  T,  being  set  about  7  feet  apart, 
and  the  top  and  sides  close-poled.  As  the  shield  advanced  the  roof  poling  was 
removed,  but  the  headtrees  were  left  to  strut  the  trench  so  formed. 

The  driving  of  the  shield  differed  in  no  respect  from  the  same  process  in 
similar  shields  already  described,  save  that  by  the  provision  of  the  central  trench 
all  the  spoil  from  the  face  was  loaded  directly  into  skips  below,  instead  of  being 
cast  on  the  platform  of  the  shield. 

The  line  of  rails  for  supplying  materials  to  the  masons,  etc.,  instead  of  being 
laid  on  the  floor  of  the  upper  half  of  the  tunnel,  was  laid  on  a  planked  platform, 
(Figs.  224  and  226),  supported  on  transverse  beams,  V,  V,  resting  on  blocks  at 
their  ends.  These  beams  were  long  enough  to  reach  across  the  full  width  of  the 
tunnel,  so  that  when  required  the  removal  of  most  of  the  soil  yet  remaining  after  the 
shield  had  passed  could  be  effected  without  interfering  with  the  working  of  the 
line  of  rails  above. 

The  masonry  arch  of  the  tunnel  was  built  under  the  protection  of  the  tail  of 
the  shield,  as  with  the  Orleans  railway  shield,  without  the  use  of  any  temporary 
timber  roof  polings,  a  space  of  about  2  or  3  inches  being  left  between  the  arch  and 
the  skin,  which  space  was  afterwards  filled  very  imperfectly  with  mortar,  the 
centres  employed  being  the  usual  iron  plate  and  angle  iron  ones,  spaced  3  feet 
4  inches  apart,  and  thirty  in  number. 

They  were  made  in  two  pieces,  the  joint  at  the  crown  being  formed  by  angle 
irons  rivetted  to  the  webs,  and  forming  a  flat  end  to  each  half  rib,  which  were 

331 


TUNNEL    SHIELDS 

fastened  together  by  bolts.  By  the  use  of  packings  in  this  joint  considerable 
adjustment  of  the  centres  was  possible. 

The  centres  were  spaced  by  cast-iron  distance  pieces,  which  served  to  distribute 
the  thrust  of  the  shield  rams,  and  in  some  cases  they  were  supported  by  props 
resting  on  footblocks  on  the  ground  beneath. 

They  appear  very  light  for  the  work,  each  half  weighing  only  about 
650  pounds  ;  they,  however,  appear  to  have  served  satisfactorily,  and  kept  their 
shape  fairly  well. 

The  laggings  were  3|  inches  thick,  and  as  usual  made  in  lengths  to  suit  the 
spacing  of  the  centres. 

The  daily  rate  of  progress  of  the  shields  where  the  advance  heading  was  used  was 
nearly  13  feet  on  the  Vincennes  section  and  about  12  feet  on  the  eleventh  section, 


FIG.  226.     METROPOLITAN  RAILWAY,  PARIS. 
The  Champigneul  Shield  :    Cross  Section  of  Tunnel  with  Centres. 

while  the  Reuilly  shield  without  a  heading  made  about  11  feet  6  inches,  the  days 
when  the  shield  was  actually  advancing  only  being  counted. 

If,  however,  the  whole  time  from  the  start  of  the  shield  to  its  final  stoppage 
be  taken,  the  figures  are  as  follows  :  Vincennes  shield,  1.1  feet  ;  shield  of  the  eighth 
section,  8  feet  9  inches  ;  and  the  Reuilly  shield,  10  feet. 

The  fact  that  the  figures  so  obtained  are  only  about  16  per  cent,  less  than 
those  obtained  by  counting  only  the  working  days  speaks  well  for  the  general 
soundness  of  the  shields'  construction.  In  shield  work  of  this  character  a  waste 
of  time  for  repairs  equal  to  25  per  cent,  of  the  whole  period  occupied  would  not  be 
excessive.1 

But  the  rate  of  advance,  however  satisfactory,  of  these  shields  hardly  com- 
pensates for  their  failure  to  perform  the  special  work  they  were  expected  to  do, 

1  The  shield  of  the  Collecteur  de  Clichy  is  said  by  M.  Philippe,  Le  Bouclier,  page  15,  to 
have  travelled  "  normally  "14  feet  9  inches  per  day.  Its  average  from  start  to  finish  was  only 
8  feet  9  inches,  a  waste  of  40  per  cent,  in  time  thus  being  shown. 

332 


THE    SHIELD    IN    MASONRY    TUNNELS 


and  which  was  the  reason  for  stipulating  for  their  use  when  the  contracts  for  the 
railway  were  drawn  up.  They  completely  justified  the  expectations  of  their 
designers  in  so  far  as  their  use  enabled  the  work  to  be  done  on  the  lengths  on  which 
they  were  employed  far  more  rapidly,  perhaps  ten  times  more  rapidly,  than  it  could 
have  been  done  with  ordinary  timbered  lengths,  with  the  same  limited  number  of 
openings  for  the  ingress  and  egress  of  material.  But  as  means  for  tunnelling  under 
the  streets  without  disturbance  of  the  city  traffic,  they  must  be  considered  as 
failures.  Indeed,  when  the  second  portion  of  the  Metropolitan  Railway  was 
commenced,  the  use  of  shields  for  tunnel  work  was  made  optional,  which,  as  the 
work  was  carried  out  under  the  supervision  of  the  same  engineers  as  the  first 
section,  would  appear  conclusive  as  to  the  lack  of  confidence  felt  in  this  method  of 
working. 

The  Champigneul  shields  were  the  most  satisfactory  of  those  employed,  but 
even  they,  if  M.  Philippe's  notes  are  exact,  left  much  to  be  desired.  The  space 
left  between  the  masonry  arch  and  the  skin  of  the  shield  was,  as  far  as  possible, 
filled  with  mortar,  "  but  often  as  the  shield  advanced,  the  ground  above  fell  in 
and  packing  became  impossible  :  the  disturbance  of  the  soil  being  transmitted  to 
the  pavement  broke  it  up . "  ( Inj  ections 
of  cement  mortar  were  tried  with  only 
moderate  success.)  "This  movement  of 
the  ground  damaged  the  roadway  very 
much  ;  with  an  average  cover  of  about 
6  feet  over  the  shield,  serious  hollows 
were  produced,  however  small  the 
amount  of  settlement  of  the  ground,  and 
its  consequent  drawing  with  the  shield  : 
settlements  of  over  30  inches  were  not 
uncommon,  with  the  result  that  the 
traffic  of  the  street  above  was  limited  to 
the  sides  clear  of  the  area  where  the 
shield  was.  This  was  written  of  the 
shields  of  the  first  section  ;  of  the  shield 
of  the  eighth  section  the  same  author  2 
says  that  with  it  the  disturbance  of  the 

street  above  was  greater  and  occurred  oftener,  a  condition  in  part  due,  however,  to 
the  yielding  character  of  the  ground  on  which  the  shield  rails  were  laid. 

It  is  not  easy  to  indicate  a  remedy  for  the  movement  of  the  ground  above 
the  shield.  Given  the  conditions,  which  obtained  alike  in  the  Orleans  Extension 
Railway  and  in  the  one  now  under  consideration,  the  roadway  must  necessarily 
be  broken  up.  Even  were  it  possible  to  avoid  leaving  any  open  space  round  the 
finished  masonry,  which,  however,  probably  had  but  little  to  do  with  the  more 
serious  dislocations  of  the  roadway,  the  movement  of  a  machine  measuring  28  feet 
across  and  23  feet  in  length,  and  propelled  by  rams  having  a  total  thrust  of  hundreds 
of  tons,  must  necessarily  break  up  the  mere  shell  of  cover  (only  2  or  3  feet  thick) 
above  it.  It  is  probable  that  the  shield  in  such  circumstances  draws  with  it  a 
certain  thickness  of  the  superincumbent  load,  and  as  the  amount  of  the  material 
so  disturbed  will  vary  almost  with  every  yard  of  advance,  undulations  of  the  road 
above  will  naturally  be  produced,  and  at  the  same  time  cavities  be  formed  above 
1  Philippe,  Le  Bouclier,  p.  154.  2  Ibid.  p.  165. 

333 


7777^J_^_V~Tl ^L^? 


\JTirtacLStage 

FIG.  227.     METROPOLITAN  RAILWAY,  PARIS. 

The  Champigneul  Shield  :     Diagram  showing  the 
Successive  Stages  of  the  Work. 


TUNNEL    SHIELDS 

the  shield,  and  this  will  probably  always  be  the  case  when  the  cover  to  the  shield 
falls  below  a  certain  thickness,  which  will  vary  with  the  nature  of  the  soil. 

When  the  portion  of  the  arch  under  the  shield  was  complete,  the  remainder 
of  the  excavation  was  taken  out  in  sections,  as  shown  in  Fig.  227.  The  third  and 
fourth  stages  were  removed  in  lengths,  of  about  6  feet  long  generally,  but  not 
always,  several  of  these  lengths  being  constructed  at  the  same  time  with  a  12- foot 
distance,  or  two  lengths,  between  them,  and  the  side  walls  built  under  the  srch 
already  erected.  The  bottom  was  taken  out  and  the  invert  built  also  in  short 
lengths,  this  forming  the  fourth  and  last  stage  of  the  tunnel  construction.  For  the 
practical  reason  of  not  interfering  more  with  the  shield  work  than  was  necessary, 
this  last  operation  was  frequently  delayed  until  the  other  work  was  nearly  com- 
pleted. 

Shields  of  the  6th  and  yth  Sections  of  the  Paris  Metropolitan  Railway 

In  the  sixth  and  seventh  sections  of  the  railway  shields  of  the  type  Lamarre 
were  employed,  and  though  these  can  hardly  be  considered  to  have  achieved  the 
moderate  success  obtained  by  the  Champigneul  shields,  some  part  of  their  failure 
may  fairly  be  laid  to  the  deficiencies  of  the  centering  against  which,  as  in  all  tunnels 
carried  out  by  this  method,  they  bore,  and  the  rigidity  of  which  forms  an  im- 
portant element  in  the  success  of  the  system. 

The  shield  (see  Figs.  228,  229)  in  its  general  outline  resembled  those  already 
described,  but  its  length  over  all  at  the  top  was  only  19  feet  9  inches  as  against 
23  feet  2  inches,  the  width  and  height  over  the  springing  line  being  of  course  the 
same.  The  distance  apart  of  the  main  frames  supporting  the  roof  was  6  feet  9 
inches,  and  to  that  extent  the  base  was  more  proportionate  to  the  length  of 
the  shield. 

On  the  other  hand,  however,  the  balance  of  the  shield  was  not  so  satisfactory 
by  reason  of  its  bearing  surfaces  in  the  transverse  direction  being,  not  as  in  the 
other  roof  shields  previously  described,  at  the  sides  of  the  frame,  but  under  the 
lower  member  of  the  framework,  which,  by  its  construction,  could  not  support  the 
extreme  edges  of  the  shield  at  each  side. 

Figs.  228  and  229  show  in  outline  the  longitudinal  and  cross  sections  of  the 
Lamarre  shield.  As  already  stated,  the  general  outline  of  the  shield  is  similar  to 
other  roof  shields  ;  in  one  respect,  however,  a  considerable  variation  is  made.  The 
roof  shields  considered  hitherto  all  had  frames  consisting  essentially  of  two  elliptical 
girders,  with  horizontal  tie-girders  beneath  them  to  prevent  any  spreading  of  the 
shield.  In  the  Lamarre  shields,  the  frame  of  the  shield  consists  of  two  frames 
or  diaphragms,  A,  B,  in  which  holes  are  cut  to  give  access  to  the  face,  and  this 
arrangement  undoubtedly  gives  greater  rigidity  than  the  other  and  more  usual 
one,  and  must  prevent  any  tendency  in  the  machine  to  settle  at  the  crown. 

On  the  other  hand  the  working  area  of  the  shield,  instead  of  being  clear  of  all 
encumbrances  (except  the  machinery)  between  the  overhead  ribs  and  the  working 
platform,  is  cramped  by  the  introduction  of  the  vertical  members  of  the  frames 
to  an  extent  which  can  be  easily  seen  by  comparing  Figs.  225  and  229. 

These  frames  A,  B  do  not  extend  the  full  width  of  the  shield,  the  outside  skin 
being  extended  beyond  them  and  supported  by  gussets,  C,  C,  fixed  upon  a  plate  D, 
which  forms  the  base  of  the  shield  between  the  frames,  and  is  turned  up  to  meet  the 
skin.  The  advantage  of  this  is  not  obvious,  and  the  disadvantage  is  unfortunately 

334 


THE    SHIELD    IN    MASONRY    TUNNELS 

very  definite.  Had  the  lower  member  of  the  frames  A,  B  been  extended  to  the 
base  of  the  shield  at  V,  V,  the  shield  could  have  been  supported  for  its  full  width. 
As  constructed  the  outside  supports  of  the  shield  are  necessarily  5  feet  within  the 
extreme  limits  of  the  skin,  an  arrangement  which  must  diminish  the  stability,  and 
in  consequence  increase  the  difficulty  of  handling  the  shield. 

The  framing  of  the  shield  is  stiffened  by  bracings,  F,  F,  and  gussets,  G,  G,  and 
as  a  structure  the  machine  is  satisfactory.  But  the  accessory  arrangements  leave 
much  to  be  desired.  The  shield  is  supported  on  sliding  plates,  H,  H,  which  rest  on 
transverse  sleepers,  J,  J,  and  in  addition  to  the  fact  that,  by  the  construction  of 
the  frames  B,  B  the  shield  has  thus  considerable  lateral  overhang,  the  arrangement 
does  not  admit  of  the  same  accuracy  of  alignment  as  is  possible  when  the  whole  shield 
travels  on  only  two  rails  or  bearing  surfaces. 


FIG.  228.     METROPOLITAN  RAILWAY,  PARIS. 
The  Lamarre  Shield  :    Longitudinal  Section. 

A  further  defect  is  to  be  found  in  the  arrangement  whereby  the  nine  rams 
K,  K,  are  arranged,  so  that  four  of  them  are  fixed  between  the  horizontal  members 
of  the  frames  A,  B,  a  position  where  they  can  be  of  little  use,  and  which  compels 
the  encumbering  of  the  working  area  behind  the  shield  with  corresponding  horizontal 
frames  to  the  centres,  and  wedge  pieces  between  them,  since  following  the  usual 
French  practice  the  shield  rams  bear  on  the  centres  supporting  the  arch  behind. 

It  will  be  seen  from  Fig.  229  that  while  six  of  the  rams  are  fitted  between  the 
webs  of  the  frames  A  and  B,  and,  as  in  English  practice,  are  framed  between  them, 
the  three  upper  ones  bear  only  on  the  front  frame  A,  and  pass  beneath  the  back 
frame  B,  to  which  they  are  hung  by  stirrups,  L,  L. 

The  rams  are  7J  inches  in  diameter,  with  a  stroke  of  3  feet  4  inches.  They 
are  single-acting,  being  drawn  back  after  the  stroke  by  a  small  parallel  ram,  and 
could  be  worked  up  to  a  pressure  of  110  tons  each. 

The  electric  pumps  and  other  machinery  are  placed  indifferently  in  the  centre 
or  side  compartments. 

The  skeleton  centres  supporting  the  masonry  were  twenty  in  number,  and  were 

335 


TUNNEL    SHIELDS 


of  timber.  Each  was  supported  at  the  centre  by  a  king  post  9  inches  by  3  inches, 
bearing  on  a  horizontal  cross  timber,  on  the  ends  of  which  the  centres  rested,  wedges 
being  driven  under  them  to  permit  of  their  adjustment.  Between  the  centres,  and 
in  the  line  of  the  rams,  were  nine  distance  pieces,  also  of  wood,  8  inches  square. 
The  use  of  timber  centering  and  framing  proved  very  unsatisfactory  ;  every  push  of 


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the  shield  was  to  a  greater  or  less  extent  reduced  in  value  owing  to  the  yielding 
of  the  timber  work  on  which  it  bore,  and  the  ill  effects  of  such  yielding  on  the 
guidance  of  the  shield  were  accentuated  by  the  method  of  supporting  the  shield 
in  sliding  plates  bearing  on  cross  sleepers  in  and  about  the  central  part  of  the  shield 
base  instead  of  on  two  defined  tracks  at  the  sides.  - 

The  design  of  the  shield,  which  necessitated  a  solid  base  beneath  the  central 

336 


THE    SHIELD    IN    MASONRY    TUNNELS 

portion,  precluded  the  use  of  an  advance  heading  like  that  which  proved  so  use- 
ful in  Sections  1  and  8,  and  the  excavation  for  the  lower  half  of  the  tunnel  was 
done  after  the  arch  was  finished.  For  some  distance  in  Section  6  water  was  met 
with,  and  a  small  timbered  heading  about  5  feet  high  by  4  feet  wide  was  driven 
ahead  to  a  pump  in  the  Place  de  la  Concorde.  When  the  soil  was  fairly  drained 
the  side  walls  were  put  in  by  underpinning,  and  the  invert  in  successive  widths. 

The  daily  advance  was  with  both  shields  from  5  feet  6  inches  to  6  feet  6  inches 
per  day. 

Little  that  is  favourable  can  be  said  of  the  Lamarre  shield  ;  it  was  difficult  to 
guide,  it  was  slow,  it  did  not  provide  a  satisfactory  working  area  either  for  the 
miners  or  masons,  the  centres  were  unsatisfactory,  and  the  movement  of  the 
surface  of  the  ground  above  was  considerable  ;  but  this  last  feature  may,  however 
be  said  to  be  common  to  most  of  the  shields  employed  in  shallow  tunnels. 

Shields  of  the  2nd  and  3rd  Sections  of  the  Paris  Metropolitan  Railway 

These  shields  were  of  the  type  known  as  Dieudonnat,  from  the  contractor  who 
employed  them.  They  resembled,  in  construction,  the  Lamarre  shield  1  without 
the  vertical  members  in  the  frames,  and  it  must  be  said  of  them  that  they  possessed 
the  faults  of  that  machine  in  an  exaggerated  degree.  The  base  was  only  22  per 
cent,  of  the  length  of  the  roof,  no  less  than  44  per  cent,  of  which  composed  the  tail 
behind.  As  the  height  of  the  shield  was  two  and  a  half  times  the  length  of  the 
base,  the  driving  of  the  shield  on  a  uniform  gradient  would  probably  have  been 
in  any  case  almost  an  impossibility,  but  the  use  of  timber  centres  and  framing 
behind,  which  in  the  case  of  the  Lamarre  shield  gave  so  much  trouble,  was  an 
additional  handicap,  and  every  movement  of  the  shield  caused  dislocations  in  the 
ground  above  and  in  the  masonry  behind. 

Of  the  three  Dieudonnat  shields  built,  one  was  abandoned  after  travelling 
132  feet,  another  accomplished  538  feet,  and  the  third  1,076  feet. 

Shields  of  the  4th  Section  of  the  Paris  Metropolitan  Railway 

These  shields,2  known  by  the  name  Weber,  from  the  contractor  who  employed 
them,  resembled  the  Dieudonnat  shields  in  their  construction.  They  were  designed 
with  the  object  of  constructing  the  tunnel  in  mass  concrete,  and  for  the  purpose 
were  constructed  with  two  sets  of  rams,  the  one  for  pushing  against  iron  centres 
erected  behind,  and  the  other  for  compressing  at  the  same  time  the  concrete  last 
filled  in. 

The  process  proved  a  complete  failure,  and  was  abandoned  after  a  short  trial, 
both  shields  finishing  their  course  in  what  may  almost  be  described  as  open  trench, 
for  the  superincumbent  material  was  removed  for  a  width  of  about  14  feet  in  the 
centre  of  the  shields,  leaving  these  to  perform  the  work  of  holding  up  the  material 
on  the  haunches  only.  Behind  the  shields  the  centres  were  erected,  and  roof  poling 
put  in  in  the  haunches  to  allow  of  concreting  in  the  ordinary  manner  later. 

The  aggregate  distance  travelled  by  the  two  shields  was  only  459  feet,  and  for 
the  greater  portion  of  this  length  they  were  practically  useless  in  that  it  was  necessary 
to  open  out  a  trench  above,  and  so  to  create  an  interference  with  the  traffic  above 
which  it  is  the  especial  function  of  such  machines  to  render  unnecessary. 

1  For  a  sketch  of  the  Dieudonnat  shield,  see  Philippe's  Le  Bouclier,  pp.  166  and  167. 

2  For  a  sketch  of  the  Weber  shield  see  Philippe's  Le  Bouclier,  pp.  186,  187. 

337  z 


TUNNEL    SHIELDS 

General   Remarks,  Paris  Metropolitan  Railway 

The  part  played  by  the  shields  throughout  the  work  cannot  be  considered 
satisfactory.  It  is  true  that  the  rate  of  progress  obtained  with  even  the  least 
efficient  of  these  machines  was  considerably  greater  than  could  have  been  made 
by  the  "  cut-and-cover  "  process  with  the  same  amount  of  interruption  of  the 
street  traffic  above  ;  but  these  shields  failed  in  supporting  the  roadway  above  them, 
not  merely  in  comparison  with  the  circular  shields  used  in  London  (which  for  the 
most  part  have  worked  at  a  greater  depth  below  the  surface),  but  as  compared  with 
the  Chagnaud  roof  shield  used  in  making  the  Collecteur  de  Clichy  "  extra  muros  "  in 
similar  conditions  in  Paris,  while  if  compared  with  the  Tremont  Street  shield,  used 
in  identical  conditions  in  Boston,  U.S.A.,  their  performances  appear  still  more 
unsatisfactory. 

A  comparison  of  the  Clichy  shield  with  the  Champigneul  shield  shows  that 
whereas  the  length  of  the  base  under  which  the  rollers  on  which  it  moved  were  placed 
on  the  former  amounted  to  60  per  cent,  of  the  total  length  of  the  hood,  the  length 
of  bearing  of  the  latter  was  less  than  one-third  (29  percent.)  of  the  total  length  at 
the  crown,  and  this  difference  in  the  proportions  of  the  machines  goes  far  towards 
explaining  the  results  obtained  by  the  two  machines.  The  shield  with  the  narrower 
base  must,  in  the  nature  of  things,  be  more  difficult  to  push  forward  under  a 
superincumbent  mass  of  earth,  heavy  enough  to  effect  it,  and  not  thick  enough  to 
have  much  effect  as  an  arch  supporting  the  roadway,  than  one  which  is  built  with 
a  base  so  proportioned  to  the  whole  area  of  the  roof  that  any  extra  pressure  at  the 
back  or  front  of  the  latter  will  be  resisted  by  the  weight  on  the  area  immediately 
over  the  base.  This  necessary  condition  for  the  stability  of  a  shield  appears  obvious, 
but  all  the  shields  employed  on  the  Paris  Metropolitan  were  built  with  an  insuffi- 
cient length  of  base. 

Compared  with  the  Tremont  Street  shield,  the  Paris  Metropolitan  shields  have 
bases  but  little  shorter  in  relation  to  their  total  length,  but  it  must  be  remembered 
that  they  in  every  case  travelled  in  a  temporary  track  laid  on  the  floor  of  the 
excavation  and  therefore  liable  to  variation  of  level  with  every  change  in  the 
material  met  with,  and  with  every  error  made  in  the  laying  of  the  track.  The 
Tremont  Street  shield,  on  the  other  hand,  was  driven  on  previously  built  side  walls 
in  which  were  built  steel  joists  laid  to  the  required  gradient.  The  shields'  tracks 
were  therefore  practically  immovable,  and  consequently  one  contributory  cause 
of  the  Paris  shields'  failure  was  absent. 

But  in  comparing  the  two  systems  it  must  be  remembered  that  the  average 
cover  over  the  Paris  shields  was  perhaps  about  one -half  that  over  the  Tremont 
shield,  and  it  may  fairly  be  questioned  whether,  given  the  conditions  of  the  Paris 
shields,  the  Tremont  shield  would  have  done  any  better  than  them.1 

1  For  a  description  of  a  shield  recently  used  in  a  later  extension  of  the  Paris  Metropolitan 
Railway  see  next  chapter. 


338 


Chapter    X 


RECENT  TUNNELLING  WORK  CARRIED  OUT  WITH  A  SHIELD  OR 

WITH  COMPRESSED  AIR 

RECENT  TUBE  RAILWAYS  IN  LONDON — THE  ROTHEKHITHE  TUNNEL,  LONDON — GENEKAL  DE- 
SCRIPTION— VERTICAL  LOCKS  IN  THE  SHAFTS — THE  SHIELD — STEEL  BULKHEAD  IN  TUNNEL 
— THE  RIVER  DEE  TUNNEL — GENERAL  DESCRIPTION — SINKING  OF  THE  SHAFTS — COM- 
PRESSED AIR  WORK — DRIVING  OF  THE  TUNNEL — PARIS  METROPOLITAN  RAILWAY  (EX- 
TENSION)— THE  RAQUET  SHIELD — CONDITIONS  OF  WORK — DESCRIPTION  OF  THE  SHIELD — 
THE  BRACKENAGH  TUNNEL,  IRELAND — THE  HILSEA  TUNNEL,  HAMPSHIRE 

IN  addition  to  the  undertakings  referred  to  in    chapters  IV,  V,  and  VII,  the 
Greathead  shield  has  since  been  almost  continuously  at  work  since  1895  in 
London,  in  one  or  other  of  the  numerous    "  tube  "  railways  which  the  last  ten 
years  have  seen  projected. 

The  full  list  of  these  undertakings  is  given  below,  the  mileage  in  each  case  being 
that  given  in  the  Act  or  Acts  of  Parliament  authorizing  the  construction  of  the 
railway  : — 

Miles. 
City  and  South  London         ...  .....      8*25 


Waterloo  and  City 

Central  London  .... 

Baker  Street  and  Waterloo  . 

Charing  Cross,  Euston  and  Hampstead 

City  and  Brixton  .... 

Edgware  and  Hampstead      « 

Great  Northern,  Piccadilly,  and  Brompton 

Metropolitan  District  Deep  Level 

North-west  London        .      -   .          .          , 

Great  Northern  and  City 

Watford  and  Edgware 

West  Metropolitan         ,          . 


1-5 

6-25 

5-4 

8-1 

4-0 

6-0 

7-1 

4-9 

3-7 

3-5 

6-2 

2-3 


In  such  of  these  railways  as  are  already  commenced  (1905)  the  system  of 
construction  does  not  vary  save  in  details  from  that  described  in  chapter  IV,  the 
uniformity  being  due  to  the  fact  that  the  nature  of  the  material  to  be  dealt  with  is 
practically  the  same  in  every  case,  namely  London  Clay. 

Other  works  in  which  a  shield  is  employed,  which  are  now  in  course  of  con- 
struction, are  the  Rotherhithe  Tunnel,  London  ;  the  Dee  Tunnel,  near  Aberdeen  ; 
the  Paris  Metropolitan  extension  ;  the  Hilsea  Tunnel,  Portsmouth  ;  and  the  recently 
completed  Brackenagh  Tunnel,  near  Dublin. 

339 


TUNNEL    SHIELDS 

The  Rotherhithe  Tunnel  1 

This  tunnel,  work  on  which  has  only  recently  been  commenced,  will  be  the 
largest  and  most  important  work  of  its  kind  in  England,  and  abroad  the  tunnel 
under  the  harbour  at  Boston,  U.S.A.,  alone  2  equals  it  in  magnitude. 

That  tunnel,  which,  however,  was  not  built  in  one  piece,  the  shield  employed 
being  of  the  roof  or  carapace  type,  was  constructed  in  concrete  under  shield  and 
compressed  air  for  a  distance  in  all  of  5,140  feet,  and  has  a  sectional  area  (outside) 
of  600  square  feet,  while  the  Rotherhithe  tunnel  will  be  constructed  of  cast  iron 
with  a  circular  shield  and  compressed  air  for  a  length  of  3,689  feet,  and  will  have 
a  sectional  area  (outside)  of  707  square  feet. 

Its  external  diameter  will  be  30  feet,  a  size 3  hitherto  only  reached  by  some 
short  lengths  of  railway  tunnels  constructed  in  London  Clay. 

Fig.  230  shows  the  position  of  the  tunnel  and  its  approaches,  and  a  longitudinal 
section,  from  which  it  will  be  seen  that  it  is  in  close  proximity  to  Brunei's  Thames 
tunnel,4  now  a  railway  tunnel,  and,  like  it,  is  intended  to  link  up  the  populous 
districts  on  either  side  of  the  Thames,  wheeled  traffic  between  which  at  present 
must  cross  the  river  either  by  the  Tower  Bridge,  1J  miles  west  of  the  northern 
entrance  to  the  proposed  tunnel  near  Stepney  Junction,  or  by  the  Blackwall 
tunnel,  the  same  distance  as  the  crow  flies  to  the  east  of  the  same  point. 

These  crossings  are,  however,  owing  to  the  curving  of  the  river  and  the  lie 
of  the  streets,  distant  1  mile  7  furlongs  and  5  miles  respectively  from  the  main 
entrance  to  the  Surrey  Commercial  Docks,  situated  close  to  the  southern  end  of 
the  tunnel. 

The  tunnel  has,  therefore,  been  undertaken  by  the  London  County  Council 
as  a  work  of  metropolitan  importance,  and  will,  it  is  expected,  be  completed  in  from 
five  to  six  years'  time. 

The  estimated  cost  of  the  work,  including  land  and  compensation,  is  about 
£2,200,000,  of  which  £1,400,000  is  for  the  engineering  works. 

In  its  general  lines  the  design  follows  that  of  the  Blackwall  tunnel,5  built  by 
the  same  authority. 

The  total  length  of  the  tunnel  and  its  approaches  is  6,883  feet  or  T30  miles. 

Commencing  at  the  north  end,  the  distance  to  the  most  northerly  shaft  in 
Broad  Street  is  1,705  feet,  of  which  1,090  feet  is  open  approach  and  615  feet  cut-and- 
cover  work.  From  the  shaft  in  Broad  Street  to  the  most  southerly  one  at  Clarence 
Street,  Rotherhithe,  the  tunnel  is  a  cast-iron  one  and  will  be  made  with  a  shield, 
and  probably  compressed  air  will  be  necessary  throughout  the  entire  length  of 
3,689  feet. 

It  is  believed  that  the  clays  and  marls  known  to  underlie  the  surface  sands  and 
gravel  will  be  above  the  crown  of  the  tunnel  for  almost  the  entire  length  to  be 
constructed  in  iron. 

These  upper  beds  are  water-bearing,  and  as,  so  far  as  is  known,  the  clay  beds 
are  not  of  great  thickness,  the  sand  and  gravel  below  them  being  in  places  con- 

1  The  author  is  indebted  to  Maurice  Fitzmaurice,  C.M.G.,  Chief  Engineer  of  the  London 
County  Council,  and  to  Messrs.  Price  &  Reeves,  Contractors  for  the  work,  for  permission  to 
publish  some  drawings  of  this  work. 

2  See  page  312. 

3  See  page  68.     Some  Station  Tunnels  of  the  City  and  South  London  Railway  are  30 
feet  in  diameter. 

4  See  page  2.  5  See  p.   180  et  seq. 

340 


341 


TUNNEL    SHIELDS 

siderably  above  the  invert  of  the  tunnel,  it  is  likely  that  much  of  the  shield  work 
will  have  to  be  done  with  a  closed  face. 

The  portion  of  the  tunnel  under  the  river  has  a  cover  of  not  less  than  10  feet, 
and  generally  a  little  more.     From  the  results  of  trial  dredgings  it  is  expected  that 


Concfete. 


FlG.  231.   ROTHEBHITHE  TUNNEL,  LONDON. 

Cross  Section  showing  proposed  arrangement  of  Headway. 

there  will   be  a  few  feet  of  clay  or   marl   above  the   crown  of  the  tunnel  for  the 
greater  part  of  the  length  under  the  river. 

The  invert  of  the  tunnel  at  the  lowest  point  is  77  feet  8  inches  below  Trinity 
highwater  mark,  so  that  the  air  pressure  employed  is  not  likely  to  be  more  than 
30  to  35  pounds  per  square  inch.1 

1  For  the  conditions  of  work  in  compressed  air  of  the  contract,  see  page  43. 

342 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 

On  the  south  side  of  the  river  the  distance  from  the  end  of  the  cast-iron  lined 
tunnel  to  the  southern  extremity  of  the  approach  works  is  1,489  feet,  of  which 
832  feet  is  open  approach  and  657  feet  cut-and-cover  work. 

The  roadway  when  made  will  have  gradients  of  1  in  37  and  1  in  36' 71  on  the 
north  and  south  respectively,  a  gradient  of  1  in  800  being  given  under  the  river 
to  assist  in  draining  the  tunnel.  The  width  of  the  roadway  will  be  16  feet,  with 
two  footways  4  feet  8  inches  wide.  The  headway  is  to  be  18  feet  6  inches  at  the 
centre,  and  15  feet  9  inches  at  the  kerbs  (Fig.  231). 


FlG.  232.   ROTHERHITHE  TUNNEL,  LONDON. 

Sections  of  Shaft,  showing  arrangement  of  Air-locks. 

Stairways  will  be  placed  in  all  the  four  shafts. 

At  present 1  the  sinking  of  the  shaft  on  the  north  bank  of  the  river  near  the 
fish  market  is  in  progress,  and  it  is  from  this  shaft  that  the  tunnelling  operations 
will  commence. 

The  steel  caissons  for  these  shafts  resemble  those  of  the  Blackwall  and  Green- 
wich tunnels,2  and  the  method  of  sinking  them  is  the  same  as  at  the  latter  work, 


April,   1905. 


2  See  Figs.  152,  153  and  112. 
343 


TUNNEL    SHIELDS 

a  second  or  lower  air-tight  floor  being  provided.  The  arrangements  of  the  air-locks 
in  the  shaft  are  on  a  different  plan,  however,  and  are  shown  in  Figs.  232,  233, 
234  and  235. 

Figs.  232  and  233  show  the  locks  and  working  shafts  as  arranged  during  the 
sinking  of  the  caissons,  the  lower  air-tight  floor  and  the  girders  for  the  upper  one 
being  in  position. 

The  work  of  excavation  is  carried  on  under  pressure  in  the  space  below  the 
lower  floor,  on  which  two  circular  shafts  8  feet  9  inches  in  diameter  are  fixed, 
terminating  in  a  chamber  or  "  bonnet  "  resting  on  the  upper  floor  girders.  This 


FIG.  233.     ROTHERHITHE  TUNNEL,  LONDON. 
Horizontal  Section  through  Air-locks  and  Bonnet. 

upper  chamber  has  circular  ends  and  roof,  and  measures  22  feet  6  inches  by  12 
feet,  thus  affording  ample  roof  for  the  working  of  the  cages,  which  are  fitted  in  the 
working  shafts,  and  are  worked  by  hydraulic  hoists. 

From  this  chamber  access  to  the  outer  air  is  gained  through  two  horizontal 
locks,  14  feet  6  inches  long  and  5  feet  9  inches  in  diameter,  and  the  skips  of  material 
brought  up  the  shafts  from  the  bottom  of  the  caisson  are  taken  through  them  to 
cages,  in  which  they  are  lifted  clear  of  the  caisson  in  progress  of  erection  above  and 
around  them. 

This  arrangement  gives  much  more  facility  for  rapid  handling  of  the  spoil  than 
the  lock  used  at  Greenwich  and  Blackwall  (figured  on  page  202),  which  had  a 

344 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 

working  capacity  of  only  eight  to  ten  skips  per  hour,  while  the  extra  expense  of 
a  large  "  bonnet  "  over  the  cost  of  two  smaller  ones,  one  to  each  shaft,  will  doubt- 


/(     jAtan,  VTEMJeiV  JA'/uVi-A.  \ 

-i.J--T;L~~.._SflfeAS—'-...l-\,.' 


Scale. 


FlG.    234.       ROTHERHITHE    TUNNEL,    LONDON". 

Air-locks  and  Bonnet.    Section  on  line  A  A  (Fig.  233). 

less  be  repaid  by  the  increased  ease  of  working  due  to  the  increased  floor  area. 
Ladders  for  use  on  an  emergency  are  provided  in  each  shaft. 


FlG.  235.   ROTHERHITHE  TUNNEL,  LONDON. 

Air-locks  and  Bonnet.     Section  on  line  B  B  (Fig.  233). 

When  the  caissons  are  sunk  to  the  required  depth,  the  air  pressure  will  be 
taken  off,  and  the  working  shafts  between  the  upper  and  lower  floors  removed, 

345 


TUNNEL    SHIELDS 

the  cylinders  composing  them  being  subsequently  utilized  for  the  horizontal  locks 
in  the  tunnel  bulkhead. 

When  the  shield  is  erected  in  the  caisson  at  the  tunnel  level,  the  upper  floor 


FlG.  236.   ROTHEBHITHE  TUNNEL,  LONDON. 

The  Shield:    Longitudinal  Section. 

will  be  closed  and  air  pressure  again  employed  for  starting  the  tunnel  by  the  removal 
of  the  "  plug  "  in  the  caisson. 

Work  will  be  continued  by  means  of  the  locks  and  bonnet  on  the  upper  floor 

346 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 

until  the  tunnel  is  sufficiently  advanced  to  permit  of  the  erection  in  it  of  a  bulk- 
head with  horizontal  locks,  when  the  caisson  will  be  cleared  of  all  the  upper  temporary 
fittings  and  left  clear  for  the  hoisting  of  material  from  the  bottom  by  means  of 


>       5       S       "r      >       "7       '« 


FlG.  237.   ROTHEBHITHE  TUNNEL,  LONDON. 

The  Shield  :    Horizontal  Section. 

the  cages  at  first  working  between  the  top  floor  and  the  top  of  the  caisson,  as  shown 
in  dotted  lines  in  Fig.  232. 

The  cast-iron  lining  of  the  tunnel  is  shown  in  Figs.  35,  36,  37  and  231,  and  but 
for  its  exceptional  strength,  due  to  the  size  of  the  tunnel,  and  its  situation  in 

347 


TUNNEL    SHIELDS 


waterlogged  material  of  varying  density,  does  not  differ  from  the  pattern  employed 
elsewhere. 

For  a  considerable  distance  on  the  north  side  of  the  river  the  tunnel  is  laid 
out  on  a  curve  of  800  feet  radius,  and  the  cast-iron  lining  along  this  portion  will 


FlG.  238.   ROTHERHITHE  TUNNEL,  LONDON. 

The  Shield  :    Half  Front  Elevation. 

be  built  in  special  tapered  rings,  the  difference  in  the  width  of  the  20-inch  rings 
employed  being  f  inch  in  the  diameter  of  the  tunnel. 

It  is  stated  in  the  specification  of  works  that  a  special  shield  will  be  required 
for  this  portion  of  the  tunnel.  The  contractors  anticipate,  however,  that  the  shield 
they  have  designed  for  the  straight  portions  of  the  work  will  do  equally  well  for 
the  curved  length.  As  the  length  of  the  shield  is  only  three-fifths  the  diameter 

348 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 


of  the  tunnel,  and  considerable  play,  some  2  inches,  is  provided  between  the  outside 
of   the    cast-iron  lining  and  the  tail  of  the  shield,  this  opinion  will  probably  be 
justified  by  results,  unless  the  character  of  the  material  met  with  be  very  bad. 
This  shield  is  shown  in  Figs.  236,  237,  238  and  239,  and  presents  some  variation 


FlG.  239.   ROTHERHITHE  TUNNEL,  LONDON. 

The  Shield  :    Half  Back  Elevation. 

in    details  from  the  type  represented  by  the  St.  Clair,  Hudson,  and    Blackwall 
shields. 

In  length  over  all  it  is  18  feet,  the  external  diameter  of  the  cutting  edge  being 
30  feet  8  inches,  and  of  the  tail  plates  30  feet  7£  inches.  But  the  circumferential 
plates  forming  the  skin  do  not  as  usual  form  a  protecting  cylinder  which  envelopes 

349 


TUNNEL    SHIELDS 

the  entire  length  of  the  shield,  but  are  only  10  feet  long,  thus  leaving  8  feet  of 
the  outside  cylinder  composed  only  of  three  cast  steel  rings  composed  of  numerous 
segments. 

The  vertical  joints  between  these  rings  and  the  joints  between  the  segments 
of  which  each  ring  is  composed  are  planed,  and  are  provided  with  machined  slots 
in  which  are  fitted  machined  steel  key  pieces,  the  effect  of  which  is  greatly  to  increase 
the  resistance  of  the  joints  to  shearing  strain,  which,  without  these  keys,  would 
of  course  be  provided  only  by  the  bolts  joining  the  segments ;  but  the  absence  of 
the  usual  cylinder  of  skin  of  rolled  steel  plates  must,  one  would  think,  materially 


FlG.  240.   ROTHEKHITHE  TUNNEL,  LONDON. 

Bulkhead  :    Diagram  showing  Disposition  of  Framing. 

weaken  the  shield's  resistance  to  tensional  strain,  such  as  is  produced  for  instance 
by  the  cutting  edge  pushing  against  a  boulder. 

The  existence  of  similar  transverse  joints  unprotected  by  skin  plates  was,  in 
the  shields  used  in  the  St.  Glair  and  Mersey  Tunnels,  found  to  be  a  source  of 
weakness. 

The  frame  of  the  shield  is  of  exceptional  strength,  there  being  three  vertical 
and  three  horizontal  diaphragms,  all  the  latter  and  the  central  vertical  one  extending 
back  10  feet  7  inches  from  the  cutting  edge,  and  being  formed  of  several  thicknesses 
of  1-inch  plates  ri vetted  together. 

There  are  no  sliding  shutters  of  the  kind  which  proved  so  useful  at  Blackwall, 
but  numerous  face  rams,  to  pairs  of  which  plates  can  be  attached  for  holding  up 
the  face,  are  provided. 

350 


^Pressure  SLcLe. 


FlG.  241.   ROTHEKHITHE  TUNNEL,  LONDON. 

Bulkhead  :    Sectional  Plan  on  line  A  B  (Fig.  240). 


351 


TUNNEL    SHIELDS 

In  addition  are  provided  on  each  working  floor  bulkheads  (removable),  which 
are  intended  in  some  measure  to  act  as  the  "  trap  "  or  water  seal  fittings  of  the 
Vyrnwy  and  Greenwich  shields. 

The  advantage  of  these  at  different  levels  of  an  open  face  shield  is  problematic, 
and  perhaps  it  would  have  been  better  had  the  face  rams  been  placed  nearer  to 
the  cutting  edge  or  made  with  a  longer  stroke,  so  that  they  could  reach  well  beyond 
the  line  of  the  cutting  edge. 

Forty  rams  for  driving  the  shield  are  provided,  and  hydraulic  erectors  similar 
in  type  to  those  of  the  Blackwall  shield  will  be  used. 

The  contractors  propose  to  substitute  for  the  customary  masonry  bulkhead 
for  the  air-locks  in  the  tunnel,  a  steel  framed  one.  This,  so  far  as  the  author  is 
aware,  has  never  been  done  before  save  in  the  Glasgow  Harbour  Tunnel,1  which, 
however,  was  only  16  feet  in  diameter. 

The  bulkhead  consists  of  |-inch  plating  supported  on  two  main  girders  composed 
each  of  two  24-inch  joists  occupying  the  vertical  and  horizontal  diameters  of  the 
tunnel  with  smaller  joists  forming  a  framing  in  which  are  set  the  three  locks.  Fig. 
240  shows  this  frame  in  outline,  and  Fig.  241  is  a  sectional  plan  of  the  bulkhead, 
locks  and  rakers  supporting  the  bulkhead. 

These  rakers,  four  in  number,  bear  on  the  bulkhead  at  the  centre  at  the  inter- 
section of  the  main  frames,  their  rear  ends  fitting  into  the  cast-iron  lining  of  the 
tunnel  six  and  seven  rings  behind  the  bulkhead. 

At  these  rings  ties  are  fixed  across  the  tunnel  to  prevent  any  spreading  of  the 
lining  close  to  the  thrust  of  the  rakers. 

The  locks  are  placed  so  that  practically  they  are  for  their  entire  length  on  the 
outer  or  atmosphere  side  of  the  bulkhead,  thus  ensuring  that  the  cylindrical  plates 
forming  them  are  subject  to  tension  only. 

The  River  Dee  Tunnel 

This  tunnel 2  will,  when  completed,  form  part  of  the  main  outfall  sewer  con- 
veying the  sewage  and  storm  water  from  Aberdeen  to  the  sea  outlet  at  Girdleness. 
It  is  situated  between  Point  Law  on  the  north  and  Torry  on  the  south  bank  of 
the  Dee,  and  is  designed,  with  the  two  shafts  on  either  side  of  the  river,  to  form 
duplicate  inverted  syphons  to  carry  the  sewage  by  gravitation  across  the  river 
(see  Fig.  242). 

The  two  cast-iron  shafts,  each  12  feet  in  internal  diameter,  are  already  sunk, 
and  no  difficulty  was  experienced  in  the  sinking  operations,  until  the  tunnel  level 
was  reached,  where  the  fine  alluvial  clay  previously  met  with  changed  to  boulder 
clay,  a  bed  of  which,  3  or  4  feet  thick,  overlay  gravel,  which  in  turn  extended  to  a 
depth  of  about  70  feet  below  Ordnance  Datum,  where  rock  was  found.  The  con- 
crete bottom  of  the  Terry  shaft  is  58- 35  and  of  the  Point  Law  shaft  50*37  feet 
below  Ordnance  Datum. 

Immediately  above  the  boulder  clay  water  was  found  in  such  quantities  as 
entirely  to  overpower  the  pulsometer  pumps  provided,  and  ultimately  the  shafts 
were  finished  by  divers,  who  excavated  to  a  depth  of  3  feet  below  the  iron  lining 
of  the  shafts  and  filled  in  6  feet  of  3  to  1  cement  concrete,  which  was  allowed  to  set 

1  See  Fig.  98. 

2  Most   of  the  facts  and  all  the  drawings  relating  to  this  work  were  put  at  the  author's 
disposal  by  Mr.  G.  R.  G.  Conway,  the  resident  engineer  of  the  tunnel. 

352 


353 


A  A 


TUNNEL    SHIELDS 


for  a  fortnight,  when  the  shafts  were  pumped  dry  and  found  to  be  perfectly  water- 
tight. 

Simultaneously  with  the  sinking  of  the  shafts,  the  construction  of  short  lengths 
of  quay  walls  of  concrete  was  proceeded  with. 

Elsewhere  these  quay  walls  are  to  be  of  timber,  but  in  order  to  prevent  any 

interference  with  the  tunnel 
at  a  future  date,  permanent 
quay  walls  were,  by  an  ar- 
rangement come  to  between 
the  Town  Council  and  the 
Harbour  Commissioners,  built 
for  a  distance  of  about  60  feet 
on  either  side  of  the  tunnel. 
In  building  these  walls,  which 
were  composed  of  concrete 
cylinders  sunk  by  excavating 
inside,  and  loading  the  cylin- 
ders with  kentledge,  openings 
9  feet  6  inches  in  diameter 
were  left  for  the  passage  of 
the  tunnel. 

As  the  quantity  of  water 
met  in  sinking  the  Torry  shaft, 
from  which  the  driving  of  the 
tunnel  was  to  be  done,  made 
the  opening  out  of  the  shaft 
at  the  tunnel  level  impossible 
without  compressed  air,  a 
vertical  air-lock  (see  Fig.  243) 
was  fitted  in  the  shaft  about 
5  feet  above  the  top  of  the 
tunnel.  This  lock  is  only 
4  feet  6  inches  high,  but  a 
"chimney"  or  pipe  is  built 
above  it,  which  gives  suffi- 
cient space  to  admit  of  pass- 
ing rails  or  timbers  up  to  about 
15  feet  long.  The  work  of 
driving  the  tunnel  is  being 
carried  on  with  this  lock,  it 
being  rightly  thought  that  in 
a  tunnel  of  such  small  dia- 
meter, and  so  short  a  length, 
the  extra  cost  of  substituting 
a  horizontal  lock  in  the  tun- 
nel would  hardly  be  repaid  by 

the  saving  in  time  in  "  locking  through  "  which  a  horizontal  lock  would  ensure. 
The  amount  of  excavation  per  yard  run  of  tunnel  is  under  7  cubic  yards,  an  amount 
which,  at  an  ordinary  rate  of  advance,  is  well  within  the  capacity  of  a  vertical  lock. 

354 


FIG.  243.     DEE  TUNNEL,  ABEKDEEN. 
The  Torry  Shaft :    Section. 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 


The  air-compressing  plant  consists  of  two  Ingersoll-Sergeant  "  straightline  " 
piston  inlet  air  compressors,  having  16-inch  steam  and  18£  air  cylinders,  the 
capacity  of  each  being  37,000  cubic  feet  of  free  air  per  hour,  and  a  small  compressor 
for  working  the  grouting  pan  and  pumping. 

5,000  cubic  feet  of  air  per  man  per  hour  is  the  allowance  aimed  at,  and  though 
the  amount  of  C02  has  reached  as  much  as  '22  per  cent,  at  the  tunnel  face,  the  usual 
amount  is  '12  to  '14  per  cent.,  a  fairly  satisfactory  condition  of  purity.1  A  medical 
lock  for  the  treatment  of  men  affected  by  caisson  sickness  is  provided  on  the  works. 

The  tunnel  itself  is  8  feet  6  inches  in  external  and  7  feet  8  inches  in  internal 
diameter,  built  of  cast-iron  segments,  five  segments  and  a  key  making  a  ring. 

The  rings  are  18  inches  wide,  and  the  weight  of  the  iron  casing  is  2  tons  12J 
cwt.  per  yard  forward.  All  the  flanges  of  the  segments  are  machined,  a  space 


©     O     0 


FIG.  244.     DEE  TUNNEL,  ABERDEEN. 
Cast  Iron  Lining. 

being  provided  for  rust  jointing.  The  bolts,  of  which  there  are  sixty  to  each  ring, 
are  1  inch  in  diameter.  Grout  holes,  as  usual,  are  provided,  but  are  not  provided 
with  screw  plugs  ;  and,  special  castings  being  required  at  the  top  and  bottom 
of  the  tunnel  to  allow  of  a  vertical  dividing  plate  to  be  fitted  later,  the  rings  of  the 
lining  do  not  break  joint. 

The  length  of  the  completed  tunnel  will  be  344  feet  centre  to  centre  of  shafts, 
and  it  is  being  constructed  at  a  depth  of  about  41  feet  below  highwater  mark,  to 
allow  of  a  cover  of  about  4  feet  above  it  in  the  event  of  the  river  channel  ever  being 
deepened  to  the  level  shown  on  Fig.  242. 

The  gradient  is  1  in  344  towards  the  Torry  shaft. 

After  the  air-lock  was  fitted  in  the  Torry  shaft,  the  tunnel  was  commenced 
by  opening  out  the  cast-iron  lining,  and  constructing  a  chamber  from  which  to 

1  See  pages  41  and  43. 
355 


TUNNEL    SHIELDS 

start,  the  shield  having  been  erected  in  the  well  of  the  shaft  previously  to  the  fixing 
of  the  air-lock. 

The  shield  is  of  the  ordinary  Greathead  type,  and  differs  little  from  those  used 
in  clay  tunnelling  elsewhere.  The  double  plates  of  the  vertical  diaphragm  arranged 
to  break  joint  make  a  very  rigid  frame  in  spite  of  the  doorway  being  larger  in  pro- 
portion to  the  shield  than  usual  (see  Fig.  245). 

In  this  diaphragm  are  thirteen  holes,  2  inches  in  diameter,  closed  with  screw 
plugs  when  not  required.  These  are  to  allow  the  working  of  drills  from  the  back 
of  the  shield  in  the  event  of  the  cutting  edge  encountering  large  boulders  in  its 
course,  granite  blocks  being  frequently  found  in  the  alluvial  clay  of  the  district. 


FIG.  245.    DEE  TUNNEL,  ABERDEEN. 
The  Shield. 

The  working  of  the  shield  follows  the  usual  lines  of  clay  working,  which  is,  no 
doubt,  sufficiently  secure,  as  there  is,  at  the  minimum,  a  cover  of  20  feet  presumably 
of  clay  over  the  crown  of  the  tunnel. 

A  heading,  5  feet  6  inches  by  3  feet  6  inches,  is  driven  in  front  of  the  shield 
and  is  timbered  with  close-set  side  props  and  headtrees,  no  polings  being  used, 
and  no  timber  being  left  in.  The  principal  risk  in  working  in  this  manner  under 
the  river  is  that,  with  the  considerable  amount  of  clay  opened  up  in  front  of  the 
shield,  a  blow,  if  such  occurred,  would  probably  be  of  such  dimensions  that  the 
small  amount  of  air  in  a  tunnel  of  such  small  diameter  would  escape  entirely  and 
the  pressure  drop  almost  instantaneously. 

The  tunnel  work  has  now  l  proceeded  for  a  distance  of  120  feet  from  the  quay 
wall,  the  average  rate  of  progress  being  about  4£  feet  per  day  of  twenty-four  hours. 

When  the  tunnel  is  completed  an  upright  steel  diaphragm  will  be  bolted  to 

1  April,   1905. 
356 


TUNNELLING    WITH  SHIELD    OR  COMPRESSED  AIR 

the  top  and  bottom  segments,  on  which  special  flanges  are  provided  for  this  purpose, 
as  shown  in  Fig.  244,  and  the  whole  lined  with  4  to  1  cement  concrete.  The  tunnel 
will  thus  be  divided  into  two  distinct  passages,  each  of  which  will  be  connected 
with  vertical  pipes,  4  feet  6  inches  in  diameter  in  each  shaft,  thus  forming  two 
independent  syphons. 

The    necessary    machinery,  penstocks,  valves,  pumps,  etc.,  will    be  provided 
within  small  valve-houses  erected  over  the  shafts. 


Paris  Metropolitan  Extension  (1905)  x 

It  was  stated  above  2  that  the  results  obtained  by  the  employment  of  shields 
in  the  construction  of  masonry  tunnels  for  a  double  line  of  railway  in  the  first 
section  of  the  Paris  Metropolitan  Railway,  comprising  the  line  from  Vincennes  to 
the  Porte  Maillot,  did  not  appear  to  have  been  sufficiently  good  to  ensure  a  further 
trial  of  the  system  on  the  second  section  of  the  line,  under  the  Outer  Boulevards 
as  no  shields  were  employed  on  this  portion. 

There  has  recently,3  however,  been  put  to  work,  on  the  section  of  the  line 
between  Vincennes  and  the  Place  d'ltalie,  a  roof  shield  of  entirely  novel  design 
which  gave  satisfactory  results  for  a  time,  and  the  ultimate  failure  of  which  was 
due  to  conditions  which  would  probably  prove  fatal  to  any  shield  of  the  same  size. 

The  shield  commenced  its  course  in  close  sand,  and  according  to  M.  Biette, 
the  engineer  of  the  line,  very  satisfactory  results  were  obtained  for  a  distance  of 
about  660  yards,  of  which  80  yards  was  on  a  curve  of  330  feet.  The  rate  of  progress 
was  often  20 'feet  per  day,  and  although  some  tendency  to  sink  was  observed,  this 
may  perhaps  be  in  part  explained  by  the  loosening  of  the  ground  under  the  centres 
by  the  driving  of  a  central  gullet  below  them  for  the  easier  removal  of  the  spoil. 

After  this  promising  commencement  it  was  somewhat  unfortunate  that  the 
line  of  the  tunnel,  on  leaving  the  sand,  passed  through  ground  completely  faulted 
by  the  irregular  settlement  of  ancient  underground  quarries,  which  are  numerous 
in  the  southern  quarter  of  Paris. 

The  uneven  support  given  by  the  underlying  material,  consisting  in  part  of  the 
solid  rock  left  to  support  the  quarry  roofs,  and  in  part  of  the  debris  which  had  filled 
the  old  workings,  proved  too  much  for  the  system  of  centering  on  which  the  shield 
moved  ;  the  centres  settled  unequally,  as  did  those  behind  supporting  the  finished 
tunnel  arch,  and  after  some  trial  the  shield  work  was  abandoned. 

But  in  some  respects,  notably  in  the  greatly  increased  area  over  which  the 
weight  was  distributed,  the  shield  appears  more  satisfactory  than  some  of  its 
predecessors  on  the  same  line,  and  a  further  trial  of  the  machine  in  undisturbed 
soil  may  very  well  be  expected  to  give  good  results. 

The  shield  is  called  by  the  name  of  its  designer,  M.  Raquet,  one  of  the  con- 
tractors for  the  railway,  and  the  principal  feature  which  differentiates  it  from 
other  roof  shields  employed  on  the  same  railway  earlier  is  the  manner  in  which  the 
centres  for  carrying  the  masonry  tunnel  behind  the  shield  are  first  made  to  support 
the  shield  itself.  All  the  tunnelling  systems  described  in  chapters  VIII  and  IX 
have  in  common  a  framed  shield  advancing  on  a  prepared  path  or  tram  lines, 
behind  which  is  erected  a  series  of  centres  on  which  a  masonry  arch  is  turned. 

1  The  author  is  indebted   to  M.  Biette,  chief  engineer  of  the  railway,  for  the  drawings 
illustrating  this  description. 

2  Page  338.  3  April,   1905. 

357 


TUNNEL    SHIELDS 

The  Raquet  shield,  however,  has  no  framework  beneath  the  roof  :  it  is,  in 


H         O 


2 


truth,  a  roof  and  nothing  more,  and  travels  by  means  of  rollers  on  tracks  formed 
of  the  distance  pieces  between  centres  erected,  not  behind  the  shield,  but  beneath 

358 


TUNNELLING    WITH  SHIELD    OR    COMPRESSED    AIR 

it,  the  first  operation  on  a  length  of  excavation  being  removed  for  the  shield  to 
advance  being  to  erect  a  centre  on  which  it  bears  as  it  moves  forward. 

Figs.  246  and  247  give  respectively  longitudinal  and  cross  sections  of    the 


I 


shield.  In  them  the  roof  is  shown  composed  of  plates  supported  by  girders,  A,  A, 
nineteen  in  number,  and  formed  of  twin  webs  with  angle  iron  flanges  running  the 
full  length  of  the  shield,  and  braced  transversely  at  intervals  varying  from  3  feet 

359 


TUNNEL    SHIELDS 


4  inches  to  4  feet  9  inches  in  length  by  built  frames,  B,  B,  B.  Beneath  alternate 
girders  are  fixed  nine  rams,  C,  C,  C,  and  ten  sets  of  rollers,  D,  D,  D,  each  10  inches 
in  diameter. 

E,  E  are  the  centres  supporting  the  shield,  and  F,  F,  F  the  cast-iron  distance 


-^j 

"f 

Jo    o    o 

ooo.S'oooo    ol; 

1 

-^ 

\  Vi^  [• 

.1              J            /" 

/ 

3 

1  «*  £j 

'SI                             E                       'SI 

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O     0^0%  00     0     0     Of"1 

i 

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u 


FIG.  248.     PARIS  METROPOLITAN  RAILWAY. 
Raquet  Shield.    Details  of  Double-ended  Shield  Ram. 

pieces  between  them.     These  distance  pieces  have  secured  to  the  top  flange  a 
2^-inch  path  to  receive  the  rollers  D  carrying  the  roof. 

These  distance  pieces  have    at    either  end    their  upper  flanges  widened  out 
so  as  to  receive  the  feet  of  the  moveable  bearings  H,  H  of  the  hydraulic  rams,  which, 


Sectio\  flfl. 


, 


Dimcfisiona  in  Inches  and  decimals  of  on  inch 
NOTC.    The  dimensions  of  fhese  bracJie/S 

Hary  Somewhat"  fo  Suit'  different 

positions . 


FIG.  249.     PARIS  METROPOLITAN  RAILWAY. 
Raquet  Shield  :    Details  of  Bearings,  H,  H,  of  Rams  shown  in  position  in  Figs.  246  and  247. 

as  shown  in  Fig.  247,  are  placed  midway  between  adjacent  distance  pieces.  The 
moveable  bearings  H,  H,  which  are  shown  in  more  detail  in  Fig.  249,  are  ingeniously 
arranged  for  throwing  the  thrust  of  the  rams  on  to  the  line  of  the  distance  pieces, 
and  for  easy  removal  from  one  piece  to  another  as  the  shield  advances. 

360 


TUNNELLING   WITH   SHIELD    OR    COMPRESSED    AIR 


The  rams  themselves  are  of  the  two-ended  pattern,  the  greater  length  which 
this  type  involves  enabling  the  bearing  of  the  rams  on  the  roof  to  be  spread  over 
a  larger  surface  than  a  single  reversing 
ram  would  provide. 

In  Fig.  246  the  shield  is  shown  in  the 
position  which  it  occupies  relatively  to 
the  leading  centre  at  the  end  of  an  advance 
of  one  metre  (3  feet  4  inches).  When  the 
excavation  of  the  face  has  proceeded  far 
enough  to  permit  of  the  erection  of  an- 
other centre,  this  is  completed  with  its 
distance  pieces,  and  the  roof  pushed  for- 
ward on  to  it.  The  pin  securing  the  pis- 
ton of  the  ram  to  the  bearing  H  is  taken  out,  and  pressure  put  in  the  rear  half  of 

Seotian.  BB . 


in  /nc/?eJ  and  decimal*  of  an  Inch 


FIG.  250.     PARIS  METROPOLITAN  RAILWAY. 
Raquet  Shield  :    Cutting  Edge. 


FIG.  251.     PARIS  METROPOLITAN  RAILWAY. 
Raquet  Shield  :    Details  of  end  of  Cylinders  of  Hydraulic  Ram. 

the  ram  which  draws  in  the  piston,   and  so  enables  the  ram-bearing  piece  H  to 
be  advanced  one  centre,  when  it  is  again  ready  to  take  the  thrust  of  the  ram. 

The  pumps   for  the  hydraulic  power  are  fixed  in  a  hanging  platform  which 

Section  on  line  flflfl. 


r 


FIG.  252.     PARIS  METROPOLITAN  RAILWAY. 
Raquet  Shield  :    Details  of  Central  Joint  of  Hydraulic  Ram. 

361 


TUNNEL    SHIELDS 


occupies  the  centre  of  the  space  under  the  shield.     The  pipe  connexions  are  not 
shown,  but  it  will  be  seen  from  Figs.  248  and  251  that  the  main  pressure  pipe  for 

driving  the  shield  is  attached  to  the  nose  of  the  ram,  a 
somewhat  unusual  position,  while  the  reversing  pipe 
enters  the  rear  cylinder  at  the  side.  (Fig.  253.) 

A  minor  feature  of  interest  is  the  manner  in  which 
the  cutting  edge  is  perforated  to  allow  of  poling  boards 
outside  the  shield  being  pulled  forward  by  light  chains  to 
hold  up  the  roof  of  the  excavation  in  front  of  the  cutting 
edge. 

This  arrangement  would  perhaps  hardly  be  found 
satisfactory  except  in  very  compact  uniform  material, 
when,  however,  the  poling  boards  would  not  be  essential. 
The  general  arrangements  for  working  the  tunnel  do 
not  appear  to  differ  from  the  earlier  work  on  the  same 
railway. 

The  details  of  the  hydraulic  rams  shown  in  Figs.    251   and  252  represent  the 
most  recent  patterns  of  this  class  of  work  made  in  France. 

The  Brackenagh  Tunnel  l 

In  connexion  with  an  extensive  scheme  for  supplying  the  city  of  Belfast, 
Ireland,  with  water  from  the  Mourne  Mountains,  it  was  necessary  to  drive  a  tunnel 


FIG.    253.      PARIS   METRO- 
POLITAN RAILWAY. 

Raquet  Shield :  Details  of 
joint  of  Pressure  Pipe  and 
Glands  in  Cylinder  of 
Hydraulic  ram. 


FIG.  254.     BRACKENAGH  TUNNEL. 
The  Shield  :   Longitudinal  Section. 

for  the  main  aqueduct  through  the  flank  of  one  of  the  hills,  known  as  the  Brackenagh 
Tunnel. 

1  The  author  is  indebted  to  Sir.  A.  R.  Binnie  for  the  drawing  of  this  shield. 

362 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 

For  the  greater  part  of  its  length,  this  tunnel  was  constructed  in  concrete,  but 
for  a  length  of  220  yards  it  traversed  a  depression  in  the  solid  beds  forming  the  hill- 
side, which  was  filled  with  a  glacial  deposit,  much  of  which  was  water-bearing 
with  some  running  sand. 

After  an  unsuccessful  attempt  to  drive  the  tunnel  through  this  material  in 
the  ordinary  manner,  and  with  a  concrete  ring,  a  cast-iron  lining  5  feet  4  inches  in 
internal  'and  6  feet  in  external  diameter  was  adopted  instead  of  the  concrete,  but 
the  ground  proved  too  difficult  for  even  this  construction  without  the  assistance 
of  a  shield  and  of  compressed  air.  With  their  help,  however,  the  tunnel  was 
successfully  driven  through  the  running  sand  (1903). 


FIG.  255.     BRAOKENAGH  TUNNEL. 
The  Shield  :    Cross  Sections. 

The  shield  employed  is  shown  in  Fig.  225  and  was  of  the  ordinary  Greathead 
type,  the  smallness  of  the  tunnel  rendering  any  face  protection  unnecessary,  and 
indeed  almost  impossible. 

The  maximum  depth  of  the  tunnel  below  ground  was  about  95  feet. 

Hilsea  Creek  Tunnel 

This  tunnel *  has  now  been  constructed  for  the  borough  of  Portsmouth  Water 
Works  Company,  and  is  for  the  purpose  of  carrying  the  water  mains  under  Hilsea 
Creek  near  the  village  of  Cosham,  Hampshire,  into  the  district  of  Portsmouth. 

A  shaft  is  sunk  on  each  bank  of  the  creek,  and  the  tunnel  has  been  driven 
(working  from  one  shaft)  at  a  depth  of  about  43  feet  below  high-water  mark  to  the 
crown  of  tunnel,  the  total  length  from  centre  to  centre  of  the  shafts  being  600  feet 
(see  Fig.  256). 

i  The  author,  is  indebted  to  [Mr.  J.  Quick  and  Mr.  R.  W.  Hoggett,  the  engineer  and  resi- 
dent engineer  respectively,  of  the  work  for  this  brief  description  and  drawings  accompanying  it. 

363 


TUNNEL    SHIELDS 

The  shafts,  21  feet  2  inches  in  external  diameter,  are  of  cast  iron  built  up  in 
rings  3  feet  deep,  each  ring  consisting  of  eight  segments.  They  are  carried  down 
to  a  hard  chalk  foundation,  the  bottom  being  in  each  case  about  62  feet  below 
high  water.  The  material  passed  through  consisted  for  the  most  part  of  loose 
chalk  through  which  water  came  freely.  This  was  kept  under  by  pumping,  the 
plant  employed  consisting  of  two  "  Evans  "  mining  pumps  and  a  No.  8  pulsometer, 
with  a  total  capacity  of  about  130,000  gallons  per  hour. 


FIG.  256.     HILSEA  CREEK  TUNNEL,  HAMPSHIRE. 
Longitudinal  Section. 

The  shafts  were  excavated  to  the  required  depths  and  the  lining  built  up  from 
the  concrete  foundations. 

A  cast-iron  false  bottom  (cast  in  sections  and  furnished  with  key  pieces)  was 
attached  to  the  shaft  ironwork,  and  the  floor  finished  with  an  inverted  concrete 
bottom.  Within  the  cast-iron  lining,  concrete,  as  shown  on  the  drawing,  Fig. 
259,  was  placed,  the  finished  work  being  15  feet  internal  diameter. 

The  opening  for  the  tunnel  was  made  in  special  castings,  built  in  at  com- 
mencement, and  was  sufficiently  large  to  permit  of  the  passage  of  the  Greathead  shield 
employed  in  the  tunnel.  The  tunnel  itself  is  12  feet  6  inches  in  external  and  11 
feet  10  inches  in  internal  diameter,  the  iron  lining  consisting  of  rings  1  foot  8  inches 


FIG.  257.     HILSEA  CREEK  TUNNEL,  HAMPSHIRE. 
Details  of  Timbering. 

wide  and  composed  of  six  segments  and  a  key,  and  weighing  about  3  tons  per  yard 
of  tunnel. 

The  shield  is  of  the  ordinary  Greathead  type  and  is  used  very  much  in  the 
manner  adopted  in  the  Glasgow  tunnels  described  in  chapter  V ;  that  is,  its  main 
function  is  to  serve  as  a  temporary  centering  within  which  to  erect  the  cast-iron 
rings,  the  excavation  being  done  in  front  of  it. 

The  method  of  working  was  as  follows  :  A  timber  heading  with  settings  4  feet 

364 


TUNNELLING    WITH    SHIELD    OR    COMPRESSED    AIR 

apart  and  close  poled  was  first  driven  and  then  opened  out  for  a  distance  of  1 2  feet 
in  front  of  the  shield,  but  only  down  to  the  horizontal  axis  of  the  tunnel.  This 
chamber  was  supported  on  crown  bars  9  inches  square  supported  by  props  on  cills 
12  inches  square,  which  rested  on  the  undisturbed  ground  in  the  lower  half  of  the 
tunnel.  This  lower  portion  was  only  removed  for  a  length  equal  to  one  tunnel 
ring  at  a  time,  and  the  shield  then  moved  forward  again,  thus  avoiding  the  necessity 
of  timbering  the  lower  half  of  the  chamber  (Fig.  257). 

This  appears  to  be  a  useful  variation  on  the  system  employed  at  Glasgow, 
and  is 'no  doubt  rendered  possible  by  the  fact  that  the  material  passed  through 
was  loose  chalk,  and  therefore  not  liable  to  run  as  would  have  been  the  case  had 
it  been  the  sand  and  gravel  met  with  at  Glasgow. 

The  quantity  of  water  dealt  with  on  the  average  amounted  to  100,000  gallons 
per  hour,  but  on  several  occasions  this  quantity  was  considerably  exceeded. 

The  rate  of  progress  has  naturally  been  slow  compared  with  tubes  of  the  same 
size  constructed  in  tho  London  Blue  Clay,  but  after  the  system  of  timber  crown  bars 
had  been  adopted  the  progress  was  about  30  feet  per  week,  or  5  feet  per  day  of 
twenty-four  hours. 


365 


Chapter  XI 

COST    OF    THE   SHIELD 

FIRST  COST  OF  SHIELD — EXAMPLES — COST  OF  TUNNELLING  PER  YARD  FORWARD,  AND  PER 
CUBIC  YARD  OF  CONTEXT — TABLES  GIVING  DETAILS  OF  QUANTITIES  AND  PRICES — COM- 
PARISON OF  COST  OF  SMALL  TUNNELS  IN  MASONRY  OR  BRICKWORK  AND  CAST  IRON — 
INCREASE  OF  COST  DUE  TO  COMPRESSED  AIR — GANGS  OF  MINERS — RATES  OF  PAY — 
NUMBERS  OF  MEN 

THE  cost  of  tunnelling  operations  with  a  shield  with  or  without  compressed 
air  is  dependent  on  so  many  circumstances  outside  of  the  mere  cost  of  the 
machine,  and  choice  of  the  material  for  the  tunnel  lining,  that  the  experience  of 
past  work  can  only  be  accepted  as  a  guide  to  a  limited  extent.  In  the  case  of  cast- 
iron  lined  tunnels,  fairly  accurate  details  are  available  as  to  the  cost  of  works  already 
executed  ;  in  estimating  for  masonry  tunnels  to  be  built  with  a  shield,  little  help 
can  be  derived  from  previous  experience,  as  no  exact  information  is  yet  available 
relative  to  the  actual  cost  of  the  more  important  works.  , 

In  considering  the  cost  per  yard  run  of  a  tunnel  the  charge  due  to  the  first 
cost  of  the  shield  is  an  important  item,  and  this  charge  obviously  is  greater  or  less 
according  to  the  length  of  the  tunnel,  the  longer  the  travel  of  the  shield  the  less 
being  the  charge  per  yard  forward. 

An  ordinary  Greathead  shield  about  12  feet  in  diameter  weighs  complete 
about  18  tons,  and  costs  £450  or  about  £25  per  ton  ;  the  larger  shield  (Fig.  74)  of 
the  Kingsway  Subway  under  Holborn,  16  feet  in  diameter,  cost  £1,600  ;  the  station 
tunnel  shield  (Fig.  71)  of  the  London  "  Tube  "  Railways  weighs  complete  85  tons, 
and  cost  £2,400  or  about  £28  per  ton. 

Of  the  shields  used  in  water-bearing  material,  the  Mersey  Tunnel  Shield,  10 
feet  in  diameter  (Figs.  145  and  146),  cost  (with  alterations)  £1,100  ;  the  Greenwich 
Tunnel  Shield,  12  feet  6  inches  in  diameter,  weighed  complete  75  tons,  and  cost 
(with  alterations),  £2,340,  or  about  £31  per  ton  ;  the  Blackwall  Tunnel  Shield,  27 
feet  8  inches  in  diameter  (Fig.  115),  weighed  220  tons  and  cost  about  £10,000,  or 
about  £45  per  ton. 

Of  shields  used  in  masonry  tunnels  the  Tremont  Street  (Boston)  Shield  with  a 
width  over  all  of  29  feet  4  inches  (Fig.  207),  weighed  22  tons,  and  cost  £1,200,  or 
£55  per  ton. 

The  cost  of  the  Boston  Harbour  Shield  (Fig.  211)  at  the  same  cost  per  ton 
would  amount  to  £3,300. 

These  prices  include  all  the  hydraulic  rams,  etc.,  and  the  connexions  on  the 
shield,  but  not  the  service  pipes  in  the  tunnel,  the  cost  of  which  obviously  varies 
with  the  distance  travelled  by  the  shield  and  with  the  nature  of  the  power  supply. 

366 


COST   OF   THE   SHIELD 

It  will  be  seen  that  the  cost  per  ton  of  the  shields  above  named  varies  from 
£25  to  £55,  and  on  dividing  up  the  cost  of  each  machine  by  the  length  of  the  tunnel 
driven,  the  cost  of  the  work  per  yard  forward  due  to  this  alone  results  in  the  case 
of  the  Greenwich  and  Blackwall  Tunnels  as  nearly  £6  in  the  former  and  over  £9 
in  the  latter  case. 

The  cost  of  repairs  to  shields,  save  in  the  event  of  a  serious  accident  causing 
crippling  of  the  skin,  is  not  usually  heavy,  being  limited  to  renewals  of  the  packings 
and  glands  of  the  hydraulic  machinery  and  connexions. 


Cost    of    Tunnelling   with    Shield 

The  subjoined  tabular  statements  I  and  II  set  forth  the  actual  prices 
(inclusive  of  shield  and  all  plant)  of  some  typical  tunnels,  and  in  III  these  figures 
are  summarized  together  with  some  figures  relating  to  earlier  works. 

For  purposes  of  comparison  the  unit  taken  in  Statement  III  is  the  cost  per 
cubic  yard  of  content  of  the  tunnel,  or  in  other  words  the  cost  per  square  yard  of 
effective  or  inside  area  per  yard  forward. 

This  is  preferable  to  taking  the  cost  per  cube  yard  of  actual  excavation,  as 
owing  to  the  great  difference  in  the  thickness  of  a  masonry  as  compared  with  a 
cast-iron  lining,  a  comparison  between  the  two  systems  based  on  that  as  a  unit  would 
be  misleading. 

STATEMENT  I 

QUANTITIES  AND  COST  PER  LINEAL  YARD  OF  IRON-LINED  TUNNELS  CONSTRUCTED 
WITH  A  SHIELD  IN  LONDON  CLAY 

NOTE. — The  figures  do  not  include  the  cost  of  pointing  (as  distinct  from  caulking  where 
necessary)  the  joints  of  the  lining,  concrete  lining,  or  any  other  internal  finishing. 

(a)  Tunnel  10  feet  6  inches  in  internal  diameter  : — 

Quantity.  Price.                       £       s.       d. 
Excavation  including  removal  and  finding  shoot  or 

tip                                                                     cubic  yards   11-3  ..  18*.  ..      10     3     5 

Cast  Iron  lining  with  planed  end  joints        .        tons     2-5  .  .  £6                    15     0     0 

Wrought  Iron  in  Bolts,  etc cwt.      1-75  ..  18.9.  ..        1    11     6 

3  to  1  Lias  Lime  Grouting        .      .      .      square  yards  12  ..  2s.  ..        140 


Price  per  yard  forward .      .      £27   18   11 

Price  per  square  yard  of  effective  sectional  area,  per  vard  forward — 

=  £27-9 -s-9-62  =£2-9 

(b)  Tunnel  11  feet  8J  inches  in  internal  diameter  : — 

Quantity.  Price.  £      *.       d. 
Excavation  including  removal  and  finding  shoot  or 

tip           cubic  yards     14      .  .  18s.  .  .  12   12     0 

Cast  Iron  lining  with  planed  end  joints  .      .      .     tons  2-83      ..  £6  ..  16  19     7 

Wrought  Iron  in  Bolts,  etc cwt.    1-94      ..  18s.  ..  1    14  11 

3  to  1  Lias  lime  grouting  behind  iron        square  yards     13       ..  2s.  ..  160 


Price  per  yard  forward £32   11     6 

Price  per  square  yard  of  effective  sectional  area,  per  yard  forward — 

=  £32-575  -=-12  =£2-714 

367 


TUNNEL    SHIELDS 

(c)  Tunnel  12  feet  6  inches  in  internal  diameter  : — 

Quantity.                  Price.  £        s.     d. 

Excavation,  etc cubic  yards     16  .  .            18-9.  .  .      14     8     0 

Cast  Iron,  etc tons     3-1  .  .        £6  2s.  .  .      18   18     3 

Wrought  Iron cwt.    1-94  ..          18s.  ..        1    14  11 

Lime  Grouting square  yards      14  .  .            2s.  .  .        180 


Price  per  yard  forward    .........      £36     8     2 

Price  per  square  yard  of  effective  sectional  area,  per  yard  forward — 

=  £36-4  H- 13-63  =£2-67 


(d)  Tunnel  15  feet  in  internal  diameter  : — 

Quantity.                 Price.  £      s.      d. 

Excavation,  etc cubic  yards  22-2     . .          18s.  20     0     0 

Cast  Iron,  etc tons     5-7      .  .        £6  2s.  .  .      34  15     4 

Wrought  Iron cwt.     3-5      .  .          18s.  .  .        330 

Lime  Grouting square  yards     17       .  .            2s.  .  .        1140 

Price  per  yard  forward          £59   12     4 

Price  per  square  yard  of  effective  sectional  area,  per  yard  forward — 

=  £59-63 -f- 19-63  =£3-03 


(e)  Tunnel  21  feet  2|  inches  in  internal  diameter  : — 

Quantity.  Price.  £       s.      d. 

Excavation cubic  yards  45-16   ..  18s.  ..  40  12  10 

Cast  Iron '  tons     8-5      .  .  £6  5s.  .  .  55     5     0 

Wrought  Iron cwt.      6-5      .  .  18s.  .  .  5   17     0 

3  to  1  Lias  Lime  Grouting    ....    square  yards  23-5     .  .  2s.  .  .  270 


Price  per  yard  forward £104     1   10 

Price  per  square  yard  of  effective  sectional  area  per  yard  forward — 

=  £104-09 -=-43  =£2-42 


(f)  Tunnel  30  feet  in  internal  diameter  : — 

Quantity.                Price.  £        s.       d. 

Excavation cubic  yards     92       ..          18s.  ..      82   16     0 

Cast  Iron         tons     20       ..        £6  10s.  .  .    130     0     0 

Wrought  Iron cwt.      12       .  .          18s.  .  .      10160 

3  to  1  Lias  Lime  Grouting     ....    square  yards     34                     2s.  .  .        380 

Price  per  yard  forward .      £227     0     0 

Price  per  square  yard  of  effective  sectional  area  per  yard  forward — 

=  £227+78  =£2-91 


(g)  Shield  Chambers,  approximate  cost  of  short  lengths  of  iron-lined  tunnels  to  serve 
as  :  not  including  cost  of  headwalls  if  used  : — 

15  feet  diameter  Chamber  for  11-81  Tunnel    .........      £220     0     0 

21-0          „         ..........    £1200     0     0 

368 


COST   OF   THE    SHIELD 


STATEMENT  II 

QUANTITIES  AND  COST  PER  YARD  FORWARD  OF  SUBAQUEOUS  TUNNELS  BUILT  WITH 

A  SHIELD  AND  COMPRESSED  AIR. 

NOTE. — The  figures  do  not  include  the  cost  of  pointing  (as  distinct  from  caulking  when 
necessary)  the  joints  of  the  lining,  concrete  lining,  or  any  other  internal  finishing. 

(h)  Blackwall  Tunnel  under  River  Thames  (1892) 

Excavation  in  compressed  air  (measured  net  exter- 
nal diameter  of  iron  tunnel  tubing)  at  the  proper 
pressure  and  without  attempting  to  dry  the  sur- 
rounding soil,  and  as  far  as  possible  without  exca- 
vating more  soil  than  the  net  area  occupied  by 
iron  tubing,  so  that  there  is  no  settlement,  and  for 
which  the  contractor  is  liable,  and  cart  or  barge 
away,  including  finding  shoot  for  same.  The  con- 
tractor must  provide  and  include,  in  his  price  for 
excavation,  for  forcing  at  sufficient  pressure  and  by 
means  of  suitable  appliances  through  holes  in  iron 
tubing  with  and  including  lias  lime,  Portland 

cement  or  other  approved  composition,  and  for  all     Quantity.  Price.  £     «.    d. 

models,  medical  attendance,  etc. .      .      cubic  yards       63-6      ..          47s.      ..      149  9     2 

Note. — The  ground  line  is  about  45  feet  average  above  crown  of  tunnel.     The  bottom  of 
river  at  least  height  is  only  6  feet  above  crown  of  tunnel. 

Cast-iron  lining  2  inches  thick  in  fourteen  segments 
with  flanges  12  inches  wide  2|  inch  metal,  including 
patterns  and  weighing  1 9  tons  9  cwt.  1  quarter  per 
yard  lineal No.  1  1  .  .  £9  15.9.  .  .  189  15  2 

Ditto  in  No.  1  key  piece,  including  pattern,  and  weigh- 
ing 8  cwt.  3  quarters  1 1  pounds  .  .  .  No.  01  .  .  £9  9s.  .  .  464 

Machining  and  coating  joints  of  segments  12  inches 
wide  and  making  joint  perfectly  watertight  in  an 
approved  manner — circumferential  joint 


lin 
Ditto,  longitudinal  joint      ....         lin( 
Clearing  and  forming   holes  in  2J  inch  mete 
Ditto  in  21  inch                ...... 

eal  feet 
3al  feet 
il     No. 
No. 
.      No. 
No. 
No. 

98       .. 
45       .  . 
1151      .  . 
1751      .. 
18 
368?      .  . 
18        .  . 

Is.   lOJd. 
Is.   10W.      • 
lid.      .  . 
lid.      . 
8d.      . 

9d.      . 

9 
4 
1 
1 
0 
1 
0 

3 
4 
1 
1 
12 
3 
13 

9 
.'5 
11 
11 
0 
0 

e 

Drilling  and  tapping  for  1  1  inch  plugs  . 
j\  inch  washers         
l|  inch  screw  plugs        

inch  bolts  10£  inch  long  over  all,  weighing  about 
9  pounds  each,  including  making  watertight  in  an 
approved  manner No.  0  1751  .  .  Is.  lO^d.  .  .  16  8  6 


Total  per  yard  lineal  of  tunnel  .      .      ,      .  .£377   19'    6 

(j)  Greenwich  Footway  Tunnel  : — 

COST  OF  TUNNELLING  PER  YARD  FORWARD. 

Excavation  in  compressed  air,  including  grouting  at 

back  of  lining cubic  yards    14}  .  .          40s.  .  .      28   10     0 

Cast-iron  lining           tons     4J  .  .      £15   12s.  .  .      64     7     0 

Wrought  iron  in  bolts  and  washers        .      .       .       cwt.     1\  .  .          24s.  . .        900 

Lead  washers No.     273  . .          l%d.  ..        1   14     1J 

Grouting  holes No.     14}  .  .          lOd.  ..       0  11   10J 

Forming  watertight  joint    ....     lineal  yards     31  .  .       2s.  Qd.  .  .        453 

Total  per  yard  lineal  of  tunnel  £108     8     3 

369  13  B 


TUNNEL    SHIELDS 


STATEMENT  III 
COMPARATIVE  STATEMENT  OF  COST  OF  IRON-LINED  TUNNELS  BUILT  WITH  A  SHIELD. 

Note. — The  cost  of  internal  lining  is  not  included. 


Tunnel. 

Date. 

Internal 
Diameter. 
Feet. 

Cost  per 
Lineal  Yard 
of  Tunnel. 

£ 

Cost  per 
square  Yard 
of  internal 
section  per 
yard  forward 
£ 

Remarks. 

"Tube"  Railways  in 

1900-1905 

London 

As  per  Detail  (a)    . 

10-50 

27-90 

2-90 

In  London  Clay. 

(b)     • 

11-68 

32-575 

2-714 

,, 

(c)     • 

12-50 

36-40 

2-67 

,, 

(d)     • 

15-00 

61-216 

3-03 

„ 

(e)     • 

21-20 

104-09 

2-42 

,, 

(/)     • 

30-00 

227-00 

2-91 

,, 

Glasgow  District  Sub- 
way 

1892 

11-00 

40-00 

3-78 

In    general    in    good 
material,   but  with 

some      subaqueous 

lengths     in     which 

compressed  air  was 

used. 

Greenwich  Tunnel     . 

1899 

11-75 

108 

9-00 

In  waterbearing  ma- 

terial.   Compressed 

air        used.       Very 

heavy  lining. 

Glasgow    Harbour 

1890 

16-00 

85 

3-35 

In   w  a  t  e  r-b  earing 

Tunnel 

sand     and     gravel, 

compressed  air  used. 

Hudson  River  Tunnel 

1889 

18-00 

300 

10-61 

In    almost   fluid   silt. 

Compressed          air 

used. 

St.  Clair  River  Tunnel 

1888 

19-83 

200 

5-81 

In  clay  under  River. 

Compressed         air 

used. 

25-00 

378 

6-93  i 

I  In  w  a  t  e  r-b  e  a  r  i  ng 

Blackwall  Tunnel 

1892     . 

[_     gravel  under  River. 
Compressed         air 

used. 

I 

25-33 

315 

5-62  i 

Note. — The  figures  do  not  include  cost  of  pointing  joints  (as  distinct  from  caulking), 
concrete  lining,  or  any  internal  finishing. 

It  will  be  seen  that,  as  regards  the  tunnels  in  London  Clay,  the  cost  per  cubic 
yard  of  content  is,  as  might  be  expected,  remarkably  uniform,  gradually  decreasing 
from  rather  less  than  £3  in  the  smaller  tunnels  to  less  than  £2  9s.  in  the  21-foot 
tunnel,  rising  again  to  nearly  £3  for  the  30-foot  one,  the  increase  being  probably 
due  to  the  more  expensive  shield  for  this  tunnel  having  a  shorter  travel  in 
proportion  than  the  others. 

The  figures  relating  to  the  Glasgow  District  Subway  Tunnels  2  are  not  very 
informing,  the  general  contract  price  of  £40  per  yard  run  including  a  considerable 
amount  of  work  under  compressed  air,  though  the  major  portion  of  the  shield 
work  was  in  fairly  good  material. 

i  The  two  prices  and  diameters  are  due  to  two  different  sections  of  lining  being  employed. 

2  See  page  139. 

370 


COST   OF  THE   SHIELD 

The  cost  of  the  tunnelling  work  in  the  Greenwich  Subway  *  giving  a  result 
of  £9  per  cubic  yard  of  content  is  greater  than  appears  necessary,  but  the  cast-iron 
lining  (see  Figs.  30,  31,  32)  was  made  of  exceptional  strength,  and  the  special  washers, 
etc.,  in  the  joints  added  to  the  expense.  In  the  light  of  the  experience  gained  in 
carrying  out  the  work  the  Author  is  of  opinion  that  a  tunnel  of  the  same  size  in 
similar  conditions  could  be  built  for  £7  10s.  per  cubic  yard  of  content. 

The  Glasgow  Harbour  Tunnels,  which  were  below  the  Clyde  for  almost  their 
entire  length,  cost  about  £3  7s.  per  cube  yard  of  content,  a  price  which  appears  very 
low  compared  with  the  Greenwich  re-suits,  but  at  Glasgow  the  tunnels  were  driven 
for  the  greater  part  of  the  distance  in  boulder  clay,  and  consequently  in  much  more 
favourable  conditions,  a  portion  being  constructed  without  the  use  of  compressed 
air,  and  for  a  certain  length  it  was  possible  to  build  a  brick  lining.2  At  Greenwich 
at  no  time  could  the  air  pressure  have  been  dispensed  with,  and  for  a  long  distance 
the  work  was  in  open  ballast  which  extended  upwards  to  the  river  bed. 

For  the  Hudson  and  St.  Clair  Tunnels,  the  diameters  of  which  are  nearly  the 
same,  the  figures  (approximate  only)  show  considerable  divergence,  as  might  be 
expected  from  the  description  of  the  works  given  in  Chapter  VI.  The  former 
work  presented  difficulties  exceptional  even  in  subaqueous  work  ;  the  latter,  in 
more  favourable  circumstances  was  built  with  a  rapidity  which  is  so  far  unequalled 
for  a  subaqueous  tunnel  of  the  same  size. 

It  will  be  seen  that  the  cost  per  cubic  yard  of  content — £5  16s. — of  the  St.  Clair 
Tunnel  is  less  than  that  of  the  Blackwall  Tunnel,  where  the  heavier  lining  was  used— 
£6  18s., — and  4s.  more  than  that  of  the  lighter  lined  lengths  of  the  same  tunnel, 
and  it  may  be  added  that  the  new  Rotherhithe  Tunnel  under  the  Thames  will,  it 
is  estimated,  cost  about  £6  per  cube  yard  of  content. 

The  cost  of  Brunei's  Thames  Tunnel  is  of  interest  in  this  connexion.  Per  cubic 
yard  of  excavation  it  cost  about  £12  8s.,  and  per  cubic  yard  of  content  £25,  including 
the  cost  of  the  shafts  at  each  end. 

From  the  above  figures,  which  cover  only  the  structural  work  of  the  tunnel 
and  do  not  include  anything  in  the  nature  of  inside  lining,  formation  of  roadway, 
or  fittings  for  the  special  purpose  for  which  each  tunnel  is  designed,  nor  the  cost  of 
approaches,  whether  shafts  or  embanked  inclines,  it  appears  that  in  estimating  for 
iron-lined  tunnels  in  London  Clay,  £3  per  cubic  yard  of  content  is  a  sufficiently 
safe  price,  and  for  tunnels  in  difficult  water-bearing  material  about  £7  per  cubic 
yard  of  content  is  a  safe  covering  figure. 

For  tunnelling  in  material  of  fair  consistency,  without  compressed  air  and  in 
which  but  little  timbering  in  front  of  the  shield  is  required,  and  when  the  amount 
of  water  met  with  can  be  dealt  with  without  laying  down  an  excessive  pumping 
plant,  about  £3  10s.  to  £4  per  cubic  yard  of  content  is  a  fair  inclusive  estimate 
for  an  iron-lined  tunnel,  the  diameter  of  which  is  not  greater  than  15  or  16  feet. 

Above  that  size,  the  great  increase  in  face  timbering  for,  and  in  the  weight 
of  a  shield  for  working  in,  loose  material  charged  with  water,  makes  estimating 
very  difficult,  and  £5  per  cubic  yard  of  content  is  probably  not  too  high  a  figure. 

A  comparison  of  the  relative  cost  of  a  masonry  tunnel  built  in  timbered  lengths, 
and  of  an  iron  tvmel  built  with  a  shield,  is  of  value  to  a  certain  extent  only. 

Tunnels  in  open  water-bearing  material,  whether  actually  under  waterways  or 

not,  are  best  built  with  a  shield,  and  the  larger  the  tunnel  the  more  the  superiority 

of  the  new  system  is  shown,  even  if  any  other  be  practicable  at  all.     But  in  tunnels 

i  See  page  231.  2  See  page  152. 

371 


TUNNEL    SHIELDS 

in  good  material  where  any  water  met  with  can  be  dealt  with  without  the  use  of 
compressed  air,  and  either  shield  work  or  timber-work  is  possible,  the  choice  of 
method  will  be  determined  by  considerations  of  cost  and  convenience. 

In  one  important  and  increasing  class  of  tunnel  work,  to  which  the  previous 
sentence  applies,  it  is  especially  probable  that  the  shield  and  iron  lining  will  be 
employed  largely  in  the  future — namely,  in  tunnels  under  or  in  the  vicinity  of  large 
cities,  where  any  interference  with  property  or  interruption  of  street  traffic,  such 
as  is  inseparable  from  the  older  system  of  working,  is  now  so  strongly  resented 
that  it  is,  when  permitted  at  all,  hedged  round  with  such  stringent  conditions  as 
to  be  economically  almost  impossible. 

Although  it  is  not  probable  that  elsewhere  the  construction  of  deep  level 
"  tube  "  railways  will  be  undertaken  on  such  an  extended  scale  as  the  last  ten 
years  have  witnessed  in  London,  in  all  great  centres  of  population  it  may  be  useful 
to  employ  the  shield  and  iron-lined  system  of  tunnelling  in  the  building  of  sewers 
above  a  certain  diameter. 

Speaking  generally  the  brick  tunnel  for  diameters  up  to  10  feet  (internal)  is 
cheaper  than  an  iron  one,  with  brick  lining,  in  good  material.  Above  10  feet 
diameter  the  cost  of  either  method  is  about  the  same  up  to  15  feet  diameter,  above 
which  the  advantage  in  price  is  with  the  iron-lined  tunnel.  But  in  all  cases  the 
latter  has  the  advantage  in  speed  of  construction  and  freedom  from  risk  of  serious 
settlement. 

A  contract  for  a  brick  sewer  9  feet  in  internal  diameter  in  good  material  has 
recently  been  let  in  London  at  £23  per  lineal  yard,  or  £3  5s.  per  cubic  yard  of  content 
(exclusive  of  cost  of  shafts).  This  price  is  a  low  one  for  brick  tunnelling,  and  is 
much  lower  than  that  of  a  same  sized  sewer  having  an  iron  casing  of  10  feet  internal 
diameter.  This,  taking  the  cost  of  the  iron  casing  at  £3  per  cubic  yard  of  its  content 
and  adding  £5  10«s.  for  the  masonry  lining,1  would  cost  £30  13s.  per  yard  forward,  or 
£4  7s.  Qd.  per  cubic  yard  of  content  (complete). 

If  a  concrete  lining  only  were  employed,  an  iron  tunnel  with  a  finished  internal 
diameter  of  9  feet  would  cost  about  £27  5s.  on  the  same  basis. 

Drawings  were  actually  prepared  of  such  alternative  designs  by  the  engineer 
carrying  out  the  work  and  the  estimates  based  on  them,  the  prices  taken  being 
current  London  contract  ones  worked  out  at  £30  for  the  iron  tunnel  with  a  concrete 
and  brick  finish  and  at  £25  for  the  iron  tunnel  finished  with  concrete  only. 

It  will  be  seen  that,  however  calculated,  the  price  of  an  iron  tunnel  of  the 
diameter  (finished)  of  9  feet  is  20  per  cent,  above  that  of  one  constructed  entirely 
in  brick. 

Simultaneously  almost  with  the  above,  a  contract  for  a  brick  sewer  11  feet 
6  inches  in  diameter  was  let  by  contract  at  a  price  of  £45  per  lineal  yard  or  £3 -9 
or  £3  18s.  per  cubic  yard  of  content,  and  in  this  case  the  ground  was  known  to  con- 
sist of  disintegrated  chalk,  gravel,  and  loose  material  containing  some  water. 

In  this  case  the  contractor  found  that  the  construction  of  a  brick  tunnel  was 
out  of  the  question,  at  any  rate  at  anything  like  the  price  at  which  he  had  taken 

1  This  is  obtained  as  under  £  s.  d. 

4  to   1  Concrete  1-6  cubic  yards  at  24s.  .      ,      .    ~.      .      1  18  4 

Brickwork               1-24     „         „          „  50s.  .....      3  2  0 

Extra  on  blue  brick 

invert                 5  sup.  yards  „  2s.   6rf.  ...      .      .      .      0  12  6 

£5   12  10 
372 


COST   OF   THE   SHIELD 

the  work,  and  he  was  permitted  to  substitute  an  iron  one  of  12  feet  6  inches  internal 
diameter. 

Again,  applying  the  cost  per  cubic  yard  of  content  as  obtained  from  the  state- 
ment on  page  370,  and  adding  the  value  of  the  masonry  lining,  the  cost  of  such  a 
tunnel  comes  out  at  £46  15s.,  or  £4-05  per  cubic  yard  of  content  per  lineal  yard. 

That  this  is  an  approximate  estimate  of  the  cost  of  the  work  was  curiously 
corroborated  by  the  fact  that  among  the  tenders  originally  sent  in  was  one  which 
offered  as  an  alternative  to  the  brick  sewer  specified  an  iron  one,  the  difference 
between  the  two  prices  sent  in  by  the  firm  tendering  being  only  about  6  per  cent, 
in  favour  of  the  brick  sewer. 

Concerning  the  cost  of  brick  or  masonry  tunnels  constructed  with  a  shield, 
but  little  information  is  available.  The  tunnels  built  in  this  manner  on  the  Paris 
Metropolitan  Railway  appear  to  have  been  not  too  fortunate  in  the  design  and 
manipulation  of  the  machines  employed  to  furnish  any  statistics  of  value,  and  no 
detailed  figures  are  available  of  the  very  successful  work  carried  out  in  this  manner 
on  the  Boston  Underground  Railway. 

M.  Legouez  l  gives  the  contract  price  per  lineal  metre  of  the  Collecteur  de 
Clichy  2  extra  muros  and  the  Collecteur  de  Clichy  intra  muros,  both  built  in  masonry 
under  shields,  at  1,016  francs  and  770  francs  respectively,  or  about  £37 '33  and 
£28-25  per  lineal  yard. 

Taking  the  same  unit  as  before,  the  cost  of  the  first  was  £1-33,  and  of  the 
second  £1  per  cubic  yard  of  content.  The  latter  of  these  prices  was  confessedly 
low,  and  the  work  was  undertaken  with  a  perhaps  too  sanguine  estimate  of  the 
capabilities  of  the  new  shield,  and  even  the  higher  price  of  the  section  "  extra 
muros  "  does  not  appear  sufficient  to  return  a  profit  to  the  contractor.  But  even 
were  the  price  doubled  there  would  be  apparently  a  very  considerable  advantage 
in  favour  of  the  masonry  tunnel  built  with  a  shield  as  compared  with  that  of  the 
iron-lined  system. 

But  in  neither  case  does  M.  Legouez  give  any  information  as  to  the  financial 
result  to  the  Contractor  of  work  done  at  the  prices  of  the  tenders,  and  even  allow- 
ing for  the  somewhat  cheaper  rates  of  wages  in  Paris,  it  is  not  easy  to  think  the 
prices  quoted  are  remunerative. 

M.  Legouez  has  made  calculations  which  he  considers  demonstrated  that  the 
use  of  a  shield  in  masonry  tunnels  results  in  an  economy  of  from  20  to  400  francs 
per  lineal  metre.3 

In  general  the  cost  of  working  with  a  shield  in  compressed  air  is  about  100  per 
cent,  more  than,  or  double,  the  cost  of  the  same  sized  tunnel  where  compressed  air 
is  not  required. 

In  tunnelling  in  the  London  Clay,  compressed  air  is  sometimes  employed  as  a 
precautionary  measure  in  the  vicinity  of  heavy  and  valuable  buildings,  all  the 
other  tunnelling  operations  remaining  unchanged.  In  such  cases  the  cost  of  the 
work  in  compressed  air  is  somewhat  less  than  double  that  of  ordinary  work. 

Where  the  amount  of  air  required  is  large,  the  cost  of  compressing  it  is  not 
less  than  twopence  per  thousand  feet  of  free  air  even  when  very  large  quantities 
are  used,*  over  a  considerable  period,  and  double  or  treble  that  for  small  installa- 
tions. 

i  Le  Bouclier,  pp.  309,  328.  2  See  Fig.   183. 

3  Bouclier,  p.  425.  *  Mr.  Moir,  Proc.  I.C.E.,  vol.  cl.  p.  54. 

373 


TUNNEL    SHIELDS 
The  wages  paid  to  the  shield  gang  are  in  London  at  the  present  date  : — 


Ganger  per  hour   . 

Miner,  ditto 

Miner's  Labourer,  ditto 

Ordinary  Labourer,  ditto 

Boy,  ditto 

Shield  Driver,  ditto 

Locksman,  ditto    . 


In  Ordinary 
Tunnelling. 

s.  d. 

1     0 

9 

n 

7 
4 
91 


In  Compressed 
Air  Tunnelling. 

s.    d. 


1     0 
1     0 


In  addition  to  these  wages,  a  bonus  for  each  ring  erected  per  week  over  a  fixed 
minimum  is  usually  paid.  This  bonus  is  generally  paid  on  an  ascending  scale, 
the  amount  per  ring  increasing  with  each  ring. 

The  miners  in  ordinary  work  have  one  10-hour  shift  per  day  ;  in  compressed 
air  work,  8-hour  shifts  are  usual  when  the  pressure  is  not  more  than  30  to  35lb 
per  square  inch.2 

The  number  of  men  employed  with  some  shields  used  in  recent  English  work 
is  given  below,  the  list  including  in  the  case  of  ordinary  tunnelling  the  men 
employed  between  the  face  and  the  first  "  turnout  "  of  the  tramway,  and  in 
compressed  air  work  all  the  men  inside  the  lock. 

COMPOSITION  OF  TUNNEL  GANGS. 


Men. 

Central  London  Railway. 

Charing  Cross  and 
Hampstead. 

Greenwich 
Tunnel. 

11  ft.  Sin.  Shield. 

22  ft.  6  in.  Shield. 

Price's  Excavator 
12  ft.  6  in.  diam. 

• 

12  ft.  6  in.  Shield. 

Ganger     

1 
4 
4 

4 

1 

1 

8 
8 
4 
1 
1 

23 

1 

2 
2 
4 
1 

10 

1 
2 
2 
5 

1 
1 

Miners      

Miners'  Labourers  . 
Ordinary  Labourers 
Boy     . 

Shield  Driver      .... 
Locksman      .... 

14 

12 

Note.  —  About  sixty  men  were  in  ordinary  circumstances  employed  in  the  Blackwall 
Tunnel.  At  the  Baker  Street  and  Waterloo  Railway  Tunnel  under  the  Thames  the  gang 
numbered  thirteen. 

In  addition  to  the  ordinary  gang,  it  is  usually  advisable  in  iron-lined  tunnels 
to  have  one  or  two  extra  men  employed  some  distance  behind  the  shield  (50  to 
60  rings)  to  tighten  up  the  bolts  of  the  cast-iron  lining,  when  the  rate  of  progress 
exceeds  five  or  six  rings  per  day. 


1  A  shield  driver  is  only  employed  with  large  shields. 

2  Four-hour  shifts  were  worked  at  the  Forth  Bridge  when  the  pressure  was  37  lb.  per 


square  inch. 


374 


APPENDIX  A 

A  CHRONOLOGICAL  LIST  OF  EVENTS  CONNECTED  WITH  TUNNELLING  BY  MEANS   OF   A 

SHIELD  OR  OF  COMPRESSED  AIR 

1818.  Brunei's  shield  patent  (see  page  1). 

1825.  The  Thames  tunnel  commenced  by  Brunei  (see  page  3). 

1828.  Dr.  Colladon  said  to  have  recommended  the  use  of  compressed  air  to  Brunei  in 
his  tunnel  enterprise. 

1830.  Sir  Thomas  Cochrane's  patent  for  constructing  shafts  and  tunnels  in  water-bearing 
strata  by  means  of  compressed  air  (see  page  23). 

1839.  Compressed  air  first  actually  used  in  sinking  a  shaft  at  Chalonnes  in  the  Loire 
Valley,  France  (see  page  27). 

1841.  About  this  time  the  Potts  system  of  vacuum  working  was  first  tried. 

1842.  The  Thames  tunnel  completed. 

1849.  A  Mr.  Dunn  patented  a  tunnelling  machine,  patent  No.  12632  of  1849  (see  page  7). 
1857.  M.  Guibal's  shield  used  for  shaft  sinking  (see  page  8). 

1861.  M.  Foley  at  Argenteuil,  France,  recommended,  for  the  first  time,  re-immersion  in 
cases  of  compressed  air  sickness  (see  page  36). 

M.  Rhizas  introduced  his  system  of  movable  iron  tunnel  centres  from  which 
the  working  face  was  supported  (see  page  8). 

1864.  Mr.  Barlow's  shield  patent,  No.  2207  of  1864  (see  page  8). 

1868.  Mr.  Beach  of  New  York  patented  a  shield  resembling  the  Barlow  shield  of  1864 

(see  page  14). 

Mr.  Barlow  provisionally  patented  another  shield,  which  patent,  however,  was 
abandoned  (see  page  9). 

1869.  The  Tower  Subway  of  cast  iron  constructed  by  means  of  a  shield  under  the  River 

Thames  by  Mr.  Greathead  (see  page  11). 

The  Broadway  Subway  constructed  by  means  of  a  shield  commenced  by  Mr. 
Beach  (see  page  14). 

1870.  The  Beach  shield  tried  in  Cincinnatti  and  Cleveland,  Ohio  (see  page  16). 

1873.  The  Woolwich  tunnel  projected  by  Mr.  Greathead.     A  feature  of  his  scheme  was 
the  use  of  compressed  air  in  conjunction  with  a  shield  (see  page  16). 

1879.  A  small  tunnel  at  Antwerp  constructed  of  cast  iron  by  M.  Hersent,  in  which  for 

the  first  time  compressed  air  was  used  in  a  tunnel  (see  page  17). 

The  Hudson  Tunnel. 

The  tunnel  under  the  Hudson  River,  New  York,  of  brick  was  commenced  by 
an  American  syndicate,  Haskin's  compressed  air  system  being  employed  (see 
page  159). 

1880.  Andersen's  patent  pilot  tunnel  system  used  at  the  Hudson  River  tunnel  (see  page 

165). 

375 


APPENDIX 

1885.  A  small  footway  tunnel  constructed  at  Stockholm  by  means  of  iron  centres  and 

the  use  of  a  freezing  machine,  the  face  of  the  tunnel  being  further  protected  by 
means  of  small  iron  plates  serving  as  polings  (see  page  275). 

1886.  Mr.  Greathead  commenced  the  City  and  South  London  Railway. 

On  this  work,  in  which  the   tunnels  were  of  cast  iron,  compressed  air  was  for 
the  first  time  used  in  conjunction  with  a  shield  in  water-bearing  strata. 
Diameter  of  tunnels  10  feet  2  inches  and  10  feet  6  inches  (seepages  88  and  137). 

1888.  The  Mersey  or  Fidler's  Ferry  Tunnel,  under  the  River  Mersey,  commenced.     This 

was  constructed  with  a  cast-iron  lining  by  means  of  a  shield  and  compressed  air. 

Diameter  of  tunnel  9  feet  (see  page  224). 

The  St.  Clair  for  Sarnia  Tunnel  was  built  under  the  St.  Clair  River  with  cast-iron 
lining,  with  shield  and  compressed  air. 

Length  of  tunnel  built  with  shield,  6,000  feet.  Diameter  of  tunnel,  19  feet 
10  inches  (see  page  172). 

1889.  A  small  tunnel  with  cast-iron  lining  and  driven  by  means  of  a  shield  was  made  at 

Blackton,  in  connexion  with  Middlesbrough  Water  Works. 

Diameter  of  tunnel,  13  feet  6  inches. 

The  Hudson  tunnel  works,  abandoned  for  some  years,  were  recommenced, 
the  tunnels  being  constructed  in  cast  iron  by  means  of  a  shield  and  compressed 
air. 

Diameter  of  tunnels,  18  feet  (see  page  167). 

1890.  Glasgow  Harbour  Tunnel.     In  this  work  three  tunnels  were  driven  side  by  side 

under  the  River  Clyde.     They  were  lined  with  cast  iron,   and  shields  and  com- 
pressed air  were  employed  in  their  construction. 
Length  of  each  tunnel,  700  feet.     Diameter  of  tunnels,  16  feet  (see  page  152). 

1891.  Glasgow  Subway.     This    work    is    a    circular    underground    railway    in   Glasgow, 

6|  miles  in  length,  of  two  tunnels  with  cast-iron  linings.  Nearly  the  entire 
length  was  built  by  means  of  Greathead  shields,  and  compressed  air  was  used 
in  the  subaqueous  portions  where  the  tunnels  pass  under  the  River  Clyde. 

Diameter  of  tunnels  (single  line),  11  feet  (see  page  139). 

A  small  tunnel  was  driven  under  the  Thames  at  Kingston  by  the  Southwark 
and  Vauxhall  Water  Company.  This  tunnel  was  of  8  feet  4  inches  diameter, 
and  was  for  its  entire  length  of  540  feet  in  the  London  Clay.  A  second  tunnel 
was  driven  near  the  first  in  1901. 

1892.  The    Blackwall    Tunnel    under    the    River    Thames.     Shield    and    compressed   air 

employed. 

Length  of  tunnel  built  with  shield,  3,116  feet.  Diameter  of  tunnel,  25  feet 
(see  page  180). 

The  Siphon  de  Clichy,  a  portion  of  the  main  drainage  system  of  Paris,  was  made 
under  the  River  Seine,  near  Clichy,  by  means  of  a  shield  of  the  Greathead  pattern, 
and  with  a  lining  of  cast  iron. 

Length  of  tunnel,  1,522  feet.  Diameter  of  the  tunnel,  8  feet  2|  inches  (see 
page  157). 

Baltimore  Belt  Line, 

An  attempt  was  made  to  construct  a  short  length  of  tunnel  by  means  of  shields, 
but  was  abandoned  almost  immediately. 

376 


APPENDIX 

East  River  Gas  Tunnel,  New  York 

This  is  a  tunnel  constructed  under  the  East  River  between  Seventy-first 
Street,  NewYork,  and  Ravenswood,  Long  Island,  for  the  convenience  of  gas  mains. 
Portions  of  the  tunnel  were  built  with  an  iron  lining,  and  a  large  proportion  of 
the  total  length  was  built  with  a  shield  and  in  compressed  air. 

Total  length  of  tunnel,  2,516  feet.  Diameter  of  iron-lined  tunnel,  10  feet 
2  inches  (see  page  216). 

A  roof  shield,  made  to  travel  on  side  walls  previously  built,  was  tried  on  a 
tunnel  under  Howard  Street,  Baltimore,  U.S.A. 

1893.  The  Mound  tunnels  at  Edinburgh  were  constructed  in  connexion  with  some 
railway  extensions  of  the  North  British  Railway  Company. 

They  were  built  under  compressed  air  and  with  a  shield,  and  lined  with  cast 
iron. 

Length  of  each  tunnel,  750  feet.  Diameter  of  tunnels,  16  feet  4  inches  (see 
page  156). 

The  Melbourne  Tunnel 

The  Melbourne  (Australia)  tunnel,  which  formed  a  part  of  the  drainage  system 
of  that  city,  was  built  in  cast  iron,  brick  and  timber  linings,  by  means  of  a  shield 
of  the  Greathead  type,  and  with  compressed  air. 

This  tunnel  is  notable  for  the  serious  accident  which  occurred  in  April,  1895, 
by  which  seven  men  lost  their  lives. 

Diameter  of  tunnel,  11  feet  (see  Engineering,  November  11,  1898). 

Emmersburg  Tunnel 

This  tunnel,  near  Schaffhausen  in  Switzerland,  is  a  small  iron-lined  tunnel 
built  by  the  aid  of  a  shield  of  rudimentary  character,  consisting  of  little  more 
than  a  roof  plate  or  hood  which  slid  over  the  iron  lining. 

Diameter,  about  5  feet  vertical,  about  3  feet  6  inches  horizontal. 

The  Waterloo  and  City  Railway,  London 

This  work  consists  of  two  single -line  tunnels,  connecting  the  Waterloo  terminus 
of  the  South-Western  Railway  with  the  City  of  London.  All  the  tunnels  are 
constructed  in  cast  iron,  and  by  means  of  shields  of  the  Greathead  type.  In 
certain  parts  of  the  line  compressed  air  was  employed,  and  shields  of  a  special 
design  were  used. 

Length  of  railway,  1  mile  46  chains.  Diameter  of  tunnels,  12  feet  If  inches, 
12  feet  9  inches  ;  diameter  of  station  tunnels,  23  feet  (see  page  143). 

1895.  Tunnel  under  the  Spree  at  Berlin 

An  experimental  length  of  tunnel  was  driven  under  compressed  air 
by  shields  designed  by  M.  Mackensen,  the  lining  being  of  wrought  iron,  with 
joints  reinforced  by  plates  projecting  outside  of  the  tunnel  circumference,  some- 
what in  the  manner  of  the  Andersen  pilot  tunnel. 

Diameter  of  tunnel,  13  feet  2  inches. 

The  Collecteur  de  Clichy  "  Extra  Muros  " 

This  main  sewer,  forming  part  of  the  drainage  system  of  Paris,  was  constructed 
at  a  depth  of  a  few  feet  only  under  the  streets  of  Paris  in  a  busy  district  by  means 
of  a  shield  designed  by  M.  Chagnaud.  The  shield  was  the  first  roof  shield  or 
"  carapace  "  employed,  and  for  the  first  time  also  a  masonry  lined  tunnel  was 
constructed  behind  any  shield. 

377 


APPENDIX 

The  sewer  was  elliptical  in  shape,  being  about  19  feet  8  inches  in  inside 
horizontal  diameter  and  16  feet  5  inches  high,  the  masonry  being  nowhere 
more  than  2  feet  thick. 

Length  of  tunnel,  5,715  feet.  Dimensions  inside  masonry,  19  feet  8  inches 
by  16  feet  5  inches  (see  page  278). 

The  Collecteur  de  Clichy  "  Intra  Muros  " 

This  was  a  continuation  of  the  preceding  work,  and  in  dimensions  and  form  of 
construction  was  the  same.  The  shield  used,  however,  was  of  the  older  type, 
completely  enclosing  the  tunnel  behind  it. 

Length  of  tunnel,  8,450  feet  (see  page  285). 

The  Siphon  de  la  Concorde 

This  siphon  under  the  River  Seine  is  like  the  Siphon  de  Clichy,  a  part  of  the 
main  drainage  of  Paris,  and,  like  it,  was  constructed  by  a  Greathead  shield,  and 
with  a  circular  cast-iron  lining. 

Length  of  siphon,  780  feet.  Diameter  of  tunnel,  8  feet  2|  inches  (see  page 
158). 

1896.     The  Central  London  Railway  i 

This  railway,  6£  miles  in  length,  connects  the  western  suburbs  of  London  with 
the  city,  and  consists  of  two  single-line  tunnels  lined  with  cast  iron,  with  larger 
tunnels  at  the  stations.  Greathead  shields  were  used  throughout,  and  at  certain 
points,  as  under  the  Holborn  Viaduct  and  in  front  of  the  Bank  of  England, 
compressed  air  was  used,  as  a  precaution  against  settlement  of  the  heavy 
structures  above. 

Diameter  of  tunnels,  11  feet  8J  inches  ;  stations,  22  feet  2|  inches  (see  page 
104). 

Tremont  Street  Tunnel,  Boston 

This  is  a  masonry  tunnel  for  two  lines  of  railway,  and  forms  part  of  the  Boston 
Underground  Railway.  It  was  built  with  a  roof  shield,  of  novel  construction, 
and  moving  in  concrete  side  walls  built  in  advance  headings,  the  arch  built  within 
the  shield  being  of  brick.  In  the  brickwork  were  embedded  cast-iron  rods  to 
take  the  thrust  of  the  shield  rams. 

Width  of  tunnel,  23  feet.     Length  built  under  shield,  550  feet  (see  page  303). 

Tunnel  under  the  River  Spree,  Berlin 

In  connexion  with  a  tramway,  a  tunnel  with  a  composite  lining  of  cast  iron 
and  concrete  was  built  with  a  hooded  shield  and  compressed  air  under  the  Spree. 
It  was  not  finished  until  1899. 

Diameter  of  tunnel,  13  feet  2  inches.     Length,  1,490  feet. 

The  Ripley  Tunnel,  U.S.A. 

The  water  supply  of  Ripley  in  New  York  State,  U.S.A.,  has  on  its  main  service 
pipe  a  tunnel  constructed  with  compressed  air  and  in  brick,  and  for  a  short  length 
with  a  shield  worked  by  hand  screws. 

Length  of  the  tunnel  built  with  shield,  60  feet.     Diameter,  3  feet  6  inches. 

1897.     Siphon  de  1'Oise,  France 

This  consists  of  a  shaft  and  tunnel  on  the  line  of  a  discharge  sewer  for  irrigation 
works  in  connexion  with  the  main  drainage  system  of  Paris.  The  tunnel  passes 

1  From  this  date  onward,  numerous  "  tube  "  railways  have  been  constructed  in  the 
London  Clay.  Timnelling  work  by  means  of  a  shield  iias,  in  fact,  been  going  on  con- 
tinuously in  London  since  1896. 


APPENDIX 

under  the  River  Oise  near  its  junction  with  the  Seine.  It  was  built  for  part  of 
its  length  with  a  shield,  and  under  compressed  air. 

The  permanent  lining  consists  of  concrete  enclosed  in  a  thin  steel  plate  casing, 
this  latter  being  built  in  rings  after  the  manner  of  cast-iron  tunnels.  The  concrete 
was  built  up  on  removable  steel  plate  centres,  and  compressed  in  position  by  the 
rams  of  the  shield. 

Length  of  tunnel  built  with  shield,  919  feet.  Diameter  of  tunnel,  6  feet  8  inches 
(see  page  291). 

1898.     Collecteur  de  Bievre,  Paris 

In  connexion  with  the  extension  of  the  Orleans  Railway  along  the  quays  of 
the  Seine,  a  considerable  alteration  in  the  system  of  main  sewers  in  the  district 
was  necessary,  and  the  diversion  of  the  Bievre  outfall  sewer,  one  of  the  main 
arteries,  was  carried  out  in  masonry  by  means  of  roof  shields. 

This  sewer  for  a  part  of  its  length  was  elliptical  in  section,  the  dimensions  inside 
the  masonry  being  :  horizontal  axis,  16  feet  2  inches  ;  vertical  axis,  10  feet  10  inches. 
For  this  section  a  roof  shield  (of  the  Chagnaud  type)  was  used. 

The  remainder  was  circular  in  section,  the  internal  diameter  being  13  feet 
2  inches,  two  roof  shields  being  used,  the  one  of  the  Chagnaud  type  and  the  other 
of  the  Dieudonnat *  pattern. 

Total  length  of  sewer,  8,025  feet. 

1898.     Aqueduct  of  the  Rivers  Loing  and  Lunain,  France 

In  making  this  aqueduct,  a  tunnel  with  concrete  lining  was  constructed,  and 
a  shield  somewhat  of  the  type  used  in  the  Paris  sewers  was  employed,  but  with 
very  unsatisfactory  results,  the  tunnel  ultimately  being  completed  in  timbered 
lengths  in  the  ordinary  manner. 

Diameter  of  tunnel,  8  feet  2|  inches. 

The  Meudon  Tunnel,  France 

This  double-line  tunnel  on  the  Western  Railway  of  France  is  situated  between 
Issy  and  Viroflay,  and  was  driven  from  its  two  extremities  at  the  same  time. 
From  the  Issy  end,  ordinary  tunnel  timbering  was  used,  from  the  other  the  tunnel 
was  driven  by  means  of  a  roof  shield  similar  in  construction  to  those  used  in  the 
Orleans  and  Metropolitan  Railways  of  Paris. 

The  arch  of  the  tunnel  is  made  of  concrete  bricks,  the  side  walls  are  of  concrete 
faced  with  concrete  blocks,  and  the  invert  of  concrete.  The  shield  work  was, 
after  some  sixteen  months,  abandoned  entirely,  a  very  small  proportion  of  the 
tunnel  having  been  driven,  and  the  work  was  finished  in  the  ordinary  manner. 

Width  of  tunnel,  29  feet  6  inches.     Height  of  tunnel,  24  feet. 

Paris  Extension  of  the  Orleans  Railway 

In  the  construction  of  this  line,  which  lies  under  the  quays  on  the  south  bank 
of  the  Seine  between  the  Pont  d'Austerlitz  and  the  Pont  de  Solferino,  three  roof 
shields  were  employed,  the  aggregate  length  of  tunnel  so  constructed  being  about 
4,000  feet.  The  tunnel  is  a  double-line  one. 

No  compressed  air  was  used,  only  the  invert  being  in  water-bearing 
material. 

Width  of  tunnel,  29  feet  6  inches.  Height  of  crown  above  rails,  15  feet  7  inches 
(see  page  295). 

Waterloo  and  Baker  Street  Railway,  London 

This  railway,  3  miles  1  furlong  in  length,  connects  the  terminal  station  of  the 
1  This  shield  resembles  the  Lamarre  shield  figured  on  page  335. 

379 


APPENDIX 

South-Western  Railway  in  London  with  the  northern  districts,  and  consists  of 
two  single-line  tunnels  with  larger  tunnels  at  the  stations. 

The  greater  portion  of  the  line  was  built  with  the  ordinary  Greathead  shield, 
only  the  length  under  the  Thames  in  water-bearing  gravel  being  driven  with 
"  trap  "  shields,  and  under  compressed  air. 

Diameter  of  tunnels,  12  feet  ;  at  stations,  23  feet  (see  page  265). 

1899.     The  Greenwich  Footway  Tunnel 

This  tunnel  passes  under  the  River  Thames  at  Greenwich,  and  was  built  with 
a  "  trap  "  shield  and  under  compressed  air.  Access  to  it  is  obtained  by  lifts  and 
stairways  placed  in  shafts  35  feet  in  diameter,  and  about  60  feet  deep. 

Length  of  tunnel,  1,217  feet.  Diameter  of  tunnel,  11  feet  9  inches  (see  page 
231). 

Metropolitan  Railway  of  Paris 

Only  a  small  portion  of  this  extensive  system  was  built  by  means  of  shields 
of  the  usual  French  type,  and  for  the  most  part  the  machines  employed  were 
unsatisfactory.  The  shields  were  exclusively  used  for  masonry  tunnels  for  two 
lines  of  railway  (see  pages  324  and  357). 

Chicago  U.S.A.,   Intercepting  Sewers 

In  these  sewers,  a  shield  known  as  the  Hastings  shield  wras  used  in  1899  (En- 
gineering News,  August  7,  1899)  for  the  construction  of  a  brick-lined  tunnel  about 
20  feet  in  internal  diameter. 

Later  another  shield  was  used  in  the  Thirty-ninth  Street  Conduit,  with  com- 
pressed air.  This  seems  to  have  resembled  the  Hudson  and  Blackwall  types 
(Engineering  News,  May  28.  1903). 

Sewer  at  Worcester,  Massachusetts,  U.S.A. 

A  small  sewer  was  constructed  in  this  year  in  brickwork  through  quicksand 
by  means  of  compressed  air.  At  one  point,  wrhere  the  sewer  passed  under  a 
railway,  a  lining  made  of  pressed  steel  plates  was  employed. 

Boston   (U.S.A.)  Harbour  Tunnel 

This  tunnel,  constructed  under  Boston  Harbour,  and  forming  part  of  the  Boston 
Underground  Railway,  is  a  masonry  one  for  two  lines  of  railway.  Between 
Sumner  Street  East,  Boston,  and  Commercial  Street,  Boston,  it  was  built  under 
compressed  air,  a  roof  shield  being  employed,  which  travelled  on  concrete  side 
walls  built  in  advance  headings. 

Width  of  tunnel,  23  feet  4  laches.  Length  of  tunnel  built  under  shield,  5,000 
feet  (see  page  312). 

1901.     Lea  River  Tunnel 

In  connexion  with  an  extension  of  the  main  drainage  of  London,  a  tunnel  of 
cast  iron,  constructed  with  a  "  trap  "  shield  and  by  means  of  compressed  air, 
was  driven  under  the  River  Lea. 

Length  of  tunnel  constructed  with  shield  and  compressed  air,  1,100  feet. 
Diameter  of  tunnel,  11  feet  (see  page  259). 

Chelsea  Tunnel 

A  small  tunnel  similar  to  that  constructed  under  the  Thames  at  Kingston  was 
built  from  Chelsea  en  the  north  to  Wandsworth  on  the  south  to  connect  up  the 
pipe  systems  of  the  New  River  Water  Company;  and  of  the  Southwark  and 
Vauxhall  Water  Company. 


APPENDIX 

New  York  "  Rapid  Transit  "  Works     (in  course  of  construction) 

The  extension  of  the  underground  railway  system  of  New  York  in  this  year 
included  the  construction  of  two  tunnels  (now  in  progress)  under  the  East  Fiver 
and  Harlem  River  respectively. 

The  subaqueous  portions  of  both  consist  of  two  single-line  circular  tunnels, 
cast-iron  lined,  and  15  feet  6  inches  in  internal  diameter. 

In  the  East  River  tunnel  a  shield  with  compressed  air  is  employed,  the  total 
length  of  double  tunnel  being  6,650  feet,  and  the  maximum  depth  of  the  rails 
below  mean  high  water,  94  feet. 

The  Harlem  River  tunnel  has  a  length  of  640  feet  of  double  iron  tunnel,  and 
was  constructed  by  means  of  McEean's  system  of  piled  timber  subaqueous 
caissons  and  compressed  air. 

See  Engineering  Record,  August  22,  September  5,  December  19  and  £6,  1903  ; 
March  5  and  12,  August  20,  1904  ;  Engineering  News,  October  1  and  8,  1903  ; 
October  13  and  November  10,  1904. 

East    River    Tunnel    of    Pennsylvania    Railway,   New    York  (in   course   of  con- 
struction) 

The  portion  of  this  tunnel  under  the  East  River  between  First  Avenue, 
Manhattan  Island,  and  East  Avenue,  Queens,  consists  of  two  cast-iron  tunnels 
similar  to  the  East  River  Tunnel  of  the  New  York  Rapid  Transit  Lines. 

1903.     Hilsea  Creek  Tunnel 

This  tunnel  was  constructed  under  the  Hilsea  Creek  for  the  line  of  pipes  supplying 
water  to  Portsmouth. 

A  shield  was  used  in  its  construction,  and  it  is  lined  with  cast-iron  segments. 
Internal  diameter  of  tunnel,  11  feet  10  inches.  length,  600  feet  (see  page 
363). 

Brackenagh  Tunnel 

A  portion  of  this  tunnel,  which  forms  part  of  the  aqueduct  supplying  Belfast 
with  water  from  the  Mourne  Mountains,  was  built  with  a  shield  and  compressed 
air,  and  lined  with  cast  iron. 

Internal  diameter  of  cast-iron  tunnel,  5  feet  4  inches.  Length  of  tunnelling 
in  compressed  air,  660  feet  (see  page  362). 

1904.     Rotherhithe  Tunnel  (works  commenced  only) 

This,  the  largest  tunnel  of  its  kind  yet  undertaken,  is  to  be  driven  under  the 
River  Thames  at  Stepney,  London. 

Diameter  of  tunnel,  30  feet  (external)  (see  page  340). 

The  Hoiborn  Tunnels 

In  connexion  with  the  construction  of  the  new  street,  Kingsway,  connecting 
Hoiborn  with  the  Strand,  the  London  County  Council  constructed  a  tramway 
subway,  which  passes  under  Hoiborn  in  two  single-line  tunnels,  circular,  with 
cast-iron  linings,  and  250  feet  in  length. 

They  were  built  with  a  shield,  the  material  passed  through  being  mainly  London 
Clay,  a  little  sand  and  gravel  showing  in  the  upper  part  of  the  face. 

Diameter  of  tunnels,  15  feet  (see  page  122). 

River  Dee  Tunnel   (in  course  of  construction) 

This  forms  a  part  of  an  extension  scheme  for  the  drainage  system  of  Aberdeen. 
The  tunnel  under  the  River  Dee  is  now  in  course  of  construction  by  means  of  a 


APPENDIX 

shield  and  compressed  air.     It  is  circular,  with  a  cast-iron  lining  of  the  usual 
type. 

Diameter  of  tunnel,  7  feet  8  inches,     Length,  344  feet  (see  page  352). 

1905.     Metropolitan  Railway  of  Paris  (Southern  section) 

A  shield  known  as  the  Raquet  shield  was  employed  for  a  short  distance  on 
an  extension  of  this  railway  near  the  Place  d'ltalie  on  the  south  side  of  the  Seine 
(see  page  357). 


APPENDIX  B 

SOME  ENGLISH  PATENTS  RELATING  TO  TUNNELLING  WITH  SHIELD  AND  COMPRESSED  AIR, 

1818  TO  1904.1 

No.  4204  of  1818.     M.  J.  Brunei. 
The  shield. 

No.  6018  of  1830.     T.  Cochrane. 

Tunnelling  by  the  aid  of  compressed  air. 

No.  12632  of  1849.     S.  Dunn. 

A  shield  in  one  piece,  having  the  front  entirely  closed,  and  intended  to  drive  through 
the  ground,  no  material  being  excavated  through  the  tunnel. 

No.  2207  of  1864.     P.  W.  Barlow. 

A  shield  in  one  piece,  combined  with  cast-iron  lining  to  the  tunnel,  which  is  to  have 
grouting  behind. 

No.  770  of  1866.     R.  Morton. 
(Provisional  protection  only.) 

A  shield  in  one  piece,  with  closed  pointed  face  and  hydraulic  rams,  combined  with 
cast-iron  tunnel  lining  (no  drawing). 

Provisional  Patent  (no  number),  1868.     P.  W.  Barlow. 

A  shield  similar  to  the  patent  2207  of  1864,  but  with  a  vertical  diaphragm. 

No.  688  of  1870.     W.  R.  Lake. 

A  shield  or  movable  frame  which  can  travel  on  the  bed  of  the  waterway  in  which  the 
tunnel  or  other  work  is  to  be  laid,  and  in  which  is  an  expanding  cloth  or  canvas  which 
can  be  opened  out  under  the  shield,  thus  making  an  air  space  in  which  the  miners  can  work. 

No.  2221  of  1873.     G.  T.  Bousfield. 

A  compressed  air-lock  like  that  used  continually  from  1845  onwards. 

No.  1738  of  1874.     J.  H.  Greathead. 

A  shield  having  a  closed  face,  the  soil  in  front  of  which  is  to  be  disintegrated  by  water 
jets,  and  by  protruding  tools.  The  tunnel  to  be  lined  with  cast  iron,  or  with  moulded 
artificial  blocks.  Grouting  to  be  injected  behind  the  tunnel  lining. 

No.  2585  of  1876.     T.  Clapham  and  W.  Clapham. 
A  mechanical  erector  for  tunnel  plates. 

1  The  list  includes  all  those  English  patents  relating  to  shields  and  to  the  use  of  com- 
pressed air  in  working  chambers  or  caissons  which  the  author  has  been  able  to  find,  but  none  of 
the  numerous  patents  involving  the  use  of  compressed  air  as  a  motive  force  for  mining 
machinery  are  inserted. 

382 


APPENDIX 

No.  5,665  of  1884.     J.  H.  Gree-thead. 

A  shield,  etc.,  as  in  No.  1738  of  1874  with  some  modifications. 
No.  5221  of  1886.     J.  H.  Greathead. 

The  grouting  pan.     A  shield  similar  to  those  of  No.  1738  of  1874  and  5665  of  1884. 

No.  13,215  of  1887.     J.  H.  Greathead. 

A  shield  with  alternatively  a  central  rotary  cutter,  with  wedges  for  breaking  down 
the  face,  washing  out  pipes,  and  wedges  only,  etc. 

No.  195  of  1889.     J.  H.  Greathead. 
A  shield  with  revolving  cutter. 

No.  919  of  1889.     Louis  Coiseau. 

A  shield  divided  into  compartments  forming  separate  air  chambers  in  conjunction 
with  a  bulkhead  and  air-lock  in  the  tunnel. 

No.  19550  of  1889.     M.  J.  Jennings. 
Roof  needles. 

No.  7374  of  1890.     J.  J.  Nobbs. 

A  shield  moving  forward  in  sections,  and  having  an  envelope  composed  of  narrow 
longitudinal  plates,  capable  of  being  advanced  separately. 

No.  18,267  of  1891.     S.  Pearson  &  Son. 

A  shield  having  compartments  forming  separate  air  chambers,  and  having  in  front 
shutters  capable  of  movement  by  means  of  hydraulic  rams. 

No.  717  of  1893.     G.  Talbot. 

An  improvement  in  timbering  in  front  of  a  shield  by  the  use  of  a  movable  rib. 

No.  1445  ,of  1893.     J.  J.  Robins. 

A  combination  of  a  shield  with  a  rotary  cutter  concentrically  fixed  in  it. 

No.  12273  of  1894.     F.  H.  Poetsch. 

A  method  of  sinking  shafts  with  a  vertical  shield  and  rotary  cutter. 

No.  12575  of  1894.     P.  Kraus. 

A  shield  with  hood  advancing  on  rollers,  and  bearing  on  iron  centres  which  support 
the  tunnel  behind. 

No.  18565  of  1895.     F.  C.  Glaser. 

A  shield  having  a  projecting  roof,  the  face  being  closed,  and  a  vertical  diaphragm 
in  the  rear  enabling  work  to  be  carried  on  with  compressed  air. 

No.  622  of  1896.     H.  H.  Dalrymple  Hay. 

A  hooded  shield,  to  be  worked  in  conjunction  with  the  use  of  clay  filling  in  front. 

No.  13907  of  1896.     J.  Price. 

A  rotary  cutter  to  work  with  an  ordinary  Greathead  shield. 

No.  16970  of  1896.     H.  H.  Dalrymple  Hay. 
Gauge  for  guiding  shields. 

No.  865  of  1897.     T.  Thomson. 

A  ladder  excavator  to  work  with  an  ordinary  Greathead  shield. 

No.  6608  of  1897.     C.  Redlich. 

A  system  of  constructing  tunnels  in  lengths,  each  length  being  built  above  ground, 
and  sunk  into  position  in  an  air-tight  caisson. 

No.  9549  of  1897.     G.  Burt. 

A  rotary  cutter,  and  conveyor  to  work  in  an  ordinary  Greathead  shield. 

No.  26,804  of  1898.     W.  J.  E.  Binnie. 

Construction  of  a  tunnel  in  mass  concrete  behind  a  shield  by  means  of  movable 
centering  of  cast  iron. 

383 


APPENDIX 

No.  11219  of  1899.     T.  H.  Murphy. 

Construction  of  a  masonry  tunnel  behind  a  shield  by  building  in  the  first  place  a 
heavy  timber  skin  within  the  shield,  forming  a  temporary  tunnel  lining,  and  affording 
an  abutment  to  receive  the  thrust  of  the  shield,  instead  of  this  latter  bearing  on  green 
masonry. 

No.  11220  of  1899.     C.  G.  Hastings  and  T.  H.  Murphy. 

A  shield  with  air-tight  bulkheads,  and  having  a  mechanical  erector  behind.  The 
tail  of  the  shield  to  consist  of  separate  flexible  strips  of  metal  instead  of  one  cylindrical 
plate. 

No.  8748  of  1900.     A.  W.  Manton. 
A  rotary  cutter  for  tunnel  work. 

No.  16981  of  1900.     C.  G.  Hastings. 

A  shield  having  compartments  fitted  with  face  shutters  composed  of  movable  slats. 

No.  10045  of  1901.     A.  W.  Farnsworth. 
A  shield  with  concentric  rotary  cutter. 

No.  26153  of  1901.     C.  M.  Jacobs. 

Construction  of  tunnels  in  soft  material  such  as  silt,  by  supporting  lengths  of  cast- 
iron  tunnel  stiffened  with  longitudinal  girders  on  piers  sunk  through  the  soft  material  to 
underlying  solid  foundations. 

No.  17227  of  1902.     J.  Breuchaud. 

Construction  of  tunnels  by  means  of  a  shield  and  compressed  air,  the  shield  being 
made  to  allow  of  the  construction  underneath  it  of  pile  or  other  foundations  on  which  it 
advances,  and  on  which  the  tunnel  is  subsequently  constructed. 

No.  18423  of  1902.     J.  F.  O'Rorke. 

Construction  of  tunnels  in  soft  or  water-bearing  material  by  caissons  in  which 
successive  lengths  are  built. 

No.  23417  of  1902.     T.  Cooper. 

A  shield  with  diaphragms  to  form  a  water-seal. 
Nos.  6828,  6830,  and  6831.     D.  D.  McBean. 

A  method  of  constructing  tunnels  under  water  in  timber  casing  with  compressed  air. 


384 


INDEX 


Age  limit  for  compressed  air  work,  42 
Air,  supply  of,  per  man  per  hour,  41,  42,  47 
Airlocks     and     Bulkheads :     vertical,     Blackwall 
Tunnel,  202 

Boston  Harbour  Tunnel,  323 

City  and  S.  London  Bailway,  137 

Cochran's  lock  in  his  specification,  28 

Dee  Tunnel,  354 

Description  of  an  ordinary,  23 

Double,  Mersey  Tunnel,  229 

Glasgow  District  Subway,  139 

Glasgow  Harbour  Tunnel,  155 

Greenwich  Tunnel.  240 

Heated  by  steam,  37 

Lea  Tunnel,  23 
„          „          Vertical  lock,  261 

with  material  shoot,  Siphon  de  1'Oise,  291 

Medical  airlock,  37 

Oise,  Siphon  de  1',  291 

Rotherhithe  Tunnel,  344,  352 
Airtight  floors  in  caissons  :  Blackwall,  185 

Greenwich,  235 

Andersen's  Pilot  Tunnel  system,  165 
Antwerp  Tunnel,  17 

Compressed  air  first  used    in    tunnelling   at, 

35 
Assisted  shield,  135 

Baker  Street  and  Waterloo    Railway  :    Cast-iron 
tunnel  lining,  58 

Clay  pockets  in  front  of  shield,  269 

Compressed  air  work,  27 1 

General  description,  264 

"  Guns  "  in  shield,  269 

Hooded  shield,  as  originally  designed,  267 
as  altered,  269 
Shutters,  failure  of,  268 

Hydraulic  rams  in  shield,  268 

Quantities  per  yard  forward,  74 

Sinking  cast-iron  shaft  in  river,  265 
Baltimore  Belt  Line,  376 
Batter  of  caissons,  182,  234 
Berlin  (R.  Spree)  Tunnel,  377,  378 
Barlow,  Mr.  :    his  share  in  the  introduction  of  the 
shield  system,  19 

Patent  of  1864,  8 

Patent  of  1868,  9 
Beach,  Mr.  :   Broadway  (New  York)  shield,  14 

his  share  in  the  introduction  of  the  shield 
system,  19 

his  type   of  shield   used   at   Cincinnati   and 

Cleveland,  16 

"  Bends,"  cure  by  re-immersion,  29,  38 
Bievre,  Collecteur  de,  379 
Blackton  Tunnel,  376 
Blackwall  Tunnel :  Caissons,  181 

Cast-iron  lining,  59 

Clay  blanket  in  river,  210 

Compressed  air  sickness  at,  41 

Conditions  of  compressed  air  work,  214 

Cost  of  tunnelling,  214,  370 

Crippling  of  cutting  edge  of  shield,  208 


Blackwall  Tunnel :   Gantry  behind  shield,  201 

General  description,  180 

Grouting  of  tunnel,  213 

Horizontal  airlocks,  206 

Lowering  of  shield  into  tunnel,  1 99 

Moulding  machine  for  segments,  70 

Percentage  of  men  rejected,  as  unfit  for  work 
in  compressed  air,  42 

Poling  in  invert  of  shield,  212 

Quantities  per  yard  forward,  74,  369 

Removal  of  "  plugs  "  in  shafts,  208 

Safety  screens,  206 

Shield,  190 

cost  of,  366 

Sliding  shutters,  use  of,  212 

Tunnelling,  cost  of,  369 

Vertical  airlocks,  202 

Bolts,  for  tunnel  lining.     See  Cast-iron  lining 
Boston  (Tremont  St. )  Tunnel :    General  descrip- 
tion, 303 

Method  of  Working,  305 

Shield,  309 

Compression  bars  in  concrete  arch,  311 
Boston   (Harbour)   Tunnel :   General   description, 
312 

Compressed  air  work,  313 

Shield,  315 

Compression  bars  in  concrete  arch,  316 

Method  of  working,  318 

Cement  used  for  grouting,  321 

Concrete  as  material  of  arch,  321 

Airlocks,  323 
Brackenagh  Tunnel,  362 
"  Break  up  "  in  London  clay,  82 

in  Hudson  Tunnels,  160,  162 
Breaking  joint  in  tunnel  segments,  54,  63 
Brick  airlock,  137,  229 
Brick  invert  in  iron  tunnel,  67 
Brick  tunnels  compared  with  iron,  49,  372 
Brick  tunnel  with  shield,  14 
Brooklyn  Bridge,  compressed  air  at,  36 
Brunei,  Mr.  :  Shield  patent  1818,  1,  4,  6,  7 

Thames  Tunnel  shield,  3 

„  „    cost  of,  371 

Bulkheads  for  airlocks.     See  Airlocks 

Caisson  disease.     See  Compressed  air  sickness 
Caissons  (See  also  Shafts) :   Airtight  floors  in,  185, 

235 

Batter  on,  182,  234,  239 
Blackwall  Tunnel,  181 
Invert,  189 

Glasgow  Harbour  Tunnel,  153 
Greenwich  Tunnel,  232 
Airtight  floors,  235 
Cutting  edge,  232 
Plug,  235,  240 
Sinking,  235 
Hudson  Tunnel,  164 
Kentledge  in,  30,  185 

"  Plugs  "  for  tunnel  openings  in,  184,  235,  240 
Rochester,  30 


385 


C  C 


INDEX 


Caissons  :   Sinking  by  concussion,  186 
Cast  iron,  in  clay,  81 
Skin  friction,  186,  187 
with  grab,  187 

Water  as  lubricant  in  sinking,  186,  235 
Canvas  and  red  lead  in  joints  of  segments,  65 
Carbonic  acid  in  air,  41 
Carson  shields.     See  Boston  Tunnels 
Cast-iron  lining  to  shafts,  80 
in  London  Clay,  81 

Baker  Street  and  Waterloo  Railway,  265 
Glasgow  Harbour  Tunnels,  152 
Siphon  de  Clichy,  157 

Cast-iron  tunnel  lining  :   Advantage  of  circular,  49 
Antwerp  Tunnel  first  used  at,  17 
Baker  Street  and  Waterloo  Railway,  58 
Blackwall  Tunnel,  59 
"  Break  up  "  for,  82 
Bolts  and  boltholes,  52,  53,  63 
Casting,  method  of,  70 
Central  London  Railway,  type  of,  54 
Commencing    tunnel    operations    in    London 

Clay,  82 

compared  with  brickwork,  49,  372 
Contraction  of,  due  to  temperature,  66 
Cost  of,  367 
Dee  Tunnel,  355 
Elliptical  boltholes,  58 

Erection,  a  method  of,  without  a  shield,  131 
Glasgow  Harbour  Tunnels,  152 
Great  Northern  and  City  Railway,  66 
Greenwich  Tunnel,  63 

Combined  with  brick  invert,  67 
with  flattened  invert,  68 
Hudson  River,  166,  170 
Joints  and  packings,  53,  55,  57,  59,  64 
Key  for,  52 

Solid,  for,  60 
Proportions  of,  60 
Lea  Tunnel,  69 
Mersey  Tunnel,  228 
Mound  Tunnels,  Edinburgh,  156 
Proportions  of,  50 
Quantities  per  yard  of  tunnel  in  clay,  73,  367 

in  water-bearing  material,  74,  369 
Rigidity  of  rings  of,  61 
Rotherhithe  Tunnel,  69 
Segments  in  each  ring  of,  52 
Siphon  de  Clichy,  158 
St.  Clair  Tunnel,  64,  172 
Tower  Subway,  12 
Waterloo  and  City  Railway,  57 
Watertightness,  62,  63,  66 
Central     London     Railway     Tunnels :      Cast-iron 

tunnel  lining,  54 
Greathead  shield,  104 
Mechanical  excavator  and  shield,  109 
Quantities  per  yard  forward,  73 
Working  gangs  for  shields,  102 
Centres,  movable,  278,  287,  300 
Rziha's  iron,  8,  48 
Telford's  iron,  48 
Chagnaud  shield,  277 
Chalonnes,  compressed  air  at,  27 
Charing  Cross  and  Hampstead  Railway  shield  and 

excavator,   113 
Champigneul  shield,  327 
Advance  heading,  331 
Centres,  331 
Method  of  working,  330 
Rate  of  progress,  332 
Remarks  on,  333 
Shoes  of,  328 


Chelsea  Tunnel,  380 
Chicago  sewers  shields,  380 

City  and  South  London  Railway  :    Airlock,  137 
"  Assisted  "  Greathead  shield,  135,  137 

General  description,  75 

Greathead  shield,  88 

Type  of  station,  81 
Clay  blanket  in  river  :  Hudson  River,  171 

Blackwall,  210 

Clay  pockets  for  shield,  146,  251,  269 
Clichy,  siphon  de,  157 
Collecteur  de  Bievre,  379 

Collecteur  de  Clichy  "  extra  muros  "  :    Centres  for 
masonry,  278 

Description  of,  277 

General  remarks,  283 

Iron  polings,  282 

Method  of  working,  282 

Roof  shield,  278 

Collecteur  de  Clichy  "  intra  muros  "  :   Building  of 
arch,  288 

Description  of,  277 

Lagging  frames,  288 

Movable  centres,  287 

Reduction  of  shield,  289 

Repair  of  tail  plates  in  shield,  289 

Roof  shield,  284 

Cochrane's  compressed  air  patent,  1830,  23,  26 
Compressed  air  :    Age  limit  for  men  working  in, 
42 

at  Antwerp  and  the  Hudson  River,  35 

Arrangement  for  men,  39 

Baker  Street  and  Waterloo  Railway,  27 1 

"  Bends,"  29,  38 

Blackwall  Tunnel,  41,  214 

Boston  (Harbour)  Tunnel,  313 

Brackenagh  Tunnel,  362 

Carbonic  acid,  percentage  of,  in  air,  40,  41 

Chalonnes  coal  pit,  27 

City  and  South  London  Railway,  137 

Cochrane's  patent,  1830,  23,  26 

Conditions  of  work  necessary,  35,  39 

Dee  Tunnel,  355 

Differential  pressures,  26 

Douchy  and  Chalonnes,  29,  35 

Early  history  of,  22 

East  River  Tunnels,  216 

Examination  of  men,  39,  41,  42 

First  used  in  tunnelling  in  1879,  35 

General  rules  for  health,  40 

Glasgow  District  Railway,  139 

Glasgow  Harbour  Tunnels,  155 

Greenwich  Tunnel,  41,  47,  237,  240 

Hours  of  labour  in,  36,  37,  39 

Hudson    Tunnels    prior    to    employment    of 
shield,  159-167 

Hudson  Tunnels  with  a  shield,  167-172 

Increase  of  temperature  due  to,  47 

London  Clay,  used  in,  77,  144 

Loss  of  pressure  due  to  friction  in  pipes,  207 

Medical  airlock,  37 

,,  ,,      at  Hudson  Tunnels,  171 

Medical  examination  of  men,  39 

Mersey  tunnel,  224 

Mound  Tunnels,  Edinburgh,  156 

Percentage  of  men  rejected  as  unfit  for  work 
in  compressed  air,  39,  42 

Purification  of  air,  45 

Re-examination  of  men  necessary,  38,  43 

Re-immersion  in,  29 

Rochester  Bridge,  30 

Rotherhithe  Tunnel  works,  fixing  conditions 
of  work,  43 


386 


INDEX 


Compressed  air :  Rules  for  work  in,  39,  40,  43 

St.  Glair  Tunnel,  172 
Sickness  :     "  Bends,"   cure  by  re-immersion, 

29,  38 

Books  relating  to,  22  (footnote) 
Causes  of,  38 
Definition  of,  38 
Dr.  Jaminet  on,  36 
Dr.  Smith  on,  36 
Siphon  de  Clichy,  157 
Siphon  de  1'Oise,  290 
Specification  of  conditions  for  men's  health, 

43 

Supply  of  air  per  man  per  hour,  41,  42,  47 
Waterloo  and  City  Railway,  143 
Woolwich  shield,  16 

Compression  bars  in  concrete  arch,  311,  316 
Concrete  tunnels.     See  Masonry  tunnels 
Concrete  and  iron  (combined)  tunnel,  290 
Conditions  of  work  in  compressed  air,  39,  40,  41,  43 
Conveyor  for  shield  work,  115,  281,  287 
Cost  of  shields,  366 
Cost  of  tunnelling  with  shield,  367 
of  shield  work,  Glasgow,  156 
Curtain  plates  in  shield  :  Blackwall,  193 

Rotherhithe,  352 
Cutting  edge  of  caisson,  81,  184,  232 

of  Greathead  shield,  89 

Cylinders.     See  Hydraulic  Rams  and  Hydraulic 
Plant 

Dalrymple  Hay's  experimental  shield  hood,  145 

Guide  rods  for  shield,  101 

Shaft  shield,  8 
Dee  Tunnel :   General  description,  352 

Airlock,  354 

Cast-iron  lining  to  tunnel,  355 

Compressed  air  plant,  355 

Method  of  working,  356 

Shield,  356 

Differential  air  pressures,  26 
Douchy,  compressed  air  employed  at,  29 
Dunn's  patent  shield,  7 

East  River  (N.Y.)  Gas  Tunnel :    Air  pressure  em- 
ployed at,  216 
General  description,  215 
Iron  poling  plates  used,  216 
Shield,  219 

Erection  of,  in  compressed  air,  221 
Strengthening  brick  lining  with  cast  iron,  217 
East  River  (Pennsylvania  Railway)  Tunnel,  381 
Elliptical  boltholes  in  tunnel  segments,  58 
Emmersburg  tunnel,  377 
Erectors  for  tunnel  segments,   17,   120,   128,   169, 

179, 196 
Examination  of  men  for  compressed  air  work,  39, 

41,  42 

Excavator,  mechanical,  110,  113 
Fidler's  Ferry  Tunnel.     See  Mersey  Tunnel 
Fougerolle  shield,  284 


Glasgow  Harbour  Tonne's :  General  description,  152 
Rate  of  progress,  155 
Shaft  sinking,  153 
Shield,  154 
Gravel,  tunnelling  in,  137,  139,  143,  148,  154,  209, 

227,  249,  253,  258,  272,  274,  362 
Greathead,  J.  H.  :    his  share  in  the  introduction 

of  the  shield  system,  19 
his  professional  career,  20  (footnote) 
Greathead  grouting  pan,  94 
Greathead  shield  :    Tower  Subway,  11-14 
"  Assisted,"  135,  137,  139,  143 
Brackenagh  Tunnel,  362 

Central  London  Railway  with  hood,  145,  158 
City  and  South  London,  88,  135 
Hilsea  Tunnel,  363 
Patents,   382,  383 
Greenwich  Tunnel :    Airlock,  240 

Amount  of  air  per  man  per  hour,  41,  47 
Caissons,  232 

Airtight  floors,  235 
Cutting  edge,  232 
Erection  and  sinking  of,  235 
"  Plug,"  235 
Removal  of  "  plug,"  240 
Sinking,  rate  of,  239 
Compressed  air,  temperature  of,  47 

rejection  of  men  for  work  in,  39 
Cost  of  tunnelling,  369 
Experiments  in  purifying  air,  45 
General  description,  230 
Lead  washers  for  bolts,  53 
Moulding  machines  for,  70 
Percentage  of  carbonic  acid  in  air,  41 
Quantities  per  yard  of  tunnel,  74 
Shield,  240 

as  finally  altered,  253 
Clay  pockets  for,  251 
Clay  pockets  and  timbering,  253 
Cost  of,  and  of  working,  257,  369 
Face  rams  altered,  249 
Special  tunnel  segments,  51 
Tunnel  lining,  63 
Groutholes  in  segments,  59,  61 
Grouting,  with  Cement,  321 

by  hand  at  Tower  Subway,  14 
in  Barlow's  patent,  9 
Operation  of,  95,  101,  213 
Pan,  94 
Ribs,  96 

Guibal's  shaft  shield,  8 
"  Guns  "  in  front  of  shields,  127,  212,  269 


Friction  on  skin  of  caissons,  186,  187,  239 

Gantry,  travelling,  behind  shield,  122,  201 
Glasgow  District  Railway,  139 

Advance  timbering,  140 

Airlock,  142 

Cast-iron  lining,  joint,  54 

Compressed  air  used,  139 

Cost  of  tunnelling,  370 

Shield,  103 
Glasgow  Harbour  Tunnels  :   Airlock,  155 

Cost  of  tunnelling,  371 


Heading,  advance  central,  331 
side,  295,  305,  318 

Andersen  pilot,  165 
Hilsea  Tunnel,  363 
Holborn,  tunnels  under,  123 
Hood  used  on  shield,  Siphon  de  Clichy,  1 58 

on  Elliptical  shield,  284 

"  Hooded "   shield,   experimental,   Waterloo    and 
City  Railway,  145 

Baker  Street  and  Waterloo  Railway,  267 
Hooded  roof  shields.     See  Roof  shields. 
Hudson  (N.Y.)  Tunnels  :  Andersen's  pilot  heading, 
165 

Caisson  sinking,  164 

Cast-iron  lining,  170 

Compressed  air  work  without  a  shield,  159-167 

Cost  of  tunnelling,  370 

General  description,  159 

Medical  airlock,  37 

Quantities  of  material  per  yard  forward,  74 

387 


INDEX 


Hudson  (N.Y.)  Tunnels  :   Segment  erector,  169 

Shield  and  working,  167-172 
Hydraulic  erectors,  17,  120,  121,  128,  169,  196 
Power  for  shields,  hand  pumps,  93,  107 

Air  pumps,  108,  117 
Shield  rams,  first  used,  15 

Baker  Street  and  Waterloo  Railway,  268 

Hand  reversing,  287 

St.  Clair  Tunnel,  177 

with  auxiliary  cylinders,  177,  178 

Blaekwall  Tunnel,  195 

Blackwall  Tunnel  auxiliary,  196 

Face,  256,  263 

Greathead  shield,  91,  93,  118 

Greenwich  and  Lea  Shields,  254,  255 

Kingsway  shield,  126 

Raquet  shield,  360 

Iron  centering  for  Mass  concrete  tunnel,  291 
Lining  of  tunnels.     See  Cast-iron  lining 
Lining  (temporary)  for  brick  tunnels,  161 
Plates  as  polings,  4,  216,  274,  282 

Joints  of  tunnel  lining.     See  Chapter  III  passim 

Kentledge  in  caissons,  34,  185,  238 
Kingston  Tunnel,  376 
Kingsway  Tunnels,  123 

Lagging,  frames,  288 

Lea  Tunnel :  Airlocks,  23,  261 

Cast-iron  lining,  69 

General  description,  258 

Imitation  of  Brunei's  patent  shield,  7 

Safety  diaphragm,  26 1 

Shield,  262 
Lead,  red,  and  canvas  joints  in  segments,  65,  69 

Washers  for  tunnel  bolts,  53,  63 

Wire  for  joints,  59,  65 

Lining,  iron,  of  tunnels.     See  Cast-iron  lining 
Lock.     See  Airlock 
Loing  and  Lunain  Aqueduct,  379 
London  Clay :  Advance  heading  in,  97,  99 

"  Break  up  "  in,  82 

Commencing  tunnelling  operations  in,  82 

Conditions  of  work  in,  76 

Cost  of  tunnelling  in,  367 

Methods  of  work  in,  96 

Mining  gang  for  shield,  strength  of,  102,  122 

Movement  of,  99,  100 

Quantities  per  yard  of  tunnels  in,  73 

Piles  in  front  of  shield  in,  97,  100 

Recent  tunnels  in,  339 

Shaft  sinking  in,  78 

Masonry   tunnels   built   with   a   shield :     Boston 

Tremont  Street  (concrete),  303 
Boston  Harbour  (concrete),  312 
Collecteur  de   Clichy   "  extra   muros "    (con- 
crete blocks),  276 

"  intra  muros  "  (concrete  blocks),  284 
General  remarks,  275 
Oise,  Siphon  de  1'  (concrete),  290 
Orleans  Railway  Extension,  Paris  (stone  and 

concrete  blocks),  294 
Paris  Metropolitan   Railway   (stone   blocks) 

324,  327,  334,  337 
Medical  airlock,  37 

Supervision  in  compressed  air  work,  39,  41,  42 
Melbourne  Tunnel,  377 

Mersey  (Fidler's  Ferry)  Tunnel :    General  descrip- 
tion, 223 


Mersey  (Filler's  Ferry)  Tunnel  :  Needles  in  front 

of  shield,  227 

Quantities  per  yard  forward,  74 
Safety  diaphragm  in  invert  of  tunnel,  227 
Shield,  as  originally  constructed,  224 

as  altered,  228 
Mining  gangs  for  shield  work,  strength  of,  102,  122, 

258,  374 

Meudon  tunnel,  379 
Morton's  shield,  9 

Moulding  machine  for  tunnel  segments,  70 
Mound  Tunnels,  Edinburgh,  156 
Movable  polings  in  hood  of  shield,  286 

Needles,  use  of,  in  front  of  Mersey  Tunnel  shield, 

227 
Greenwich  shield,  249 

Oise,  Siphon  de  1',  290 

Airlock,  291 

Concrete  and  iron  lining,  291 

Shield,  290 
Orleans  Railway  extension,  294 

Masonry  work,  300 

Method  of  working,  295 

the  shield,  297 

Paris  Extension  of  Orleans  Railway.     See  Orleans 

Railway  Extension 
Paris  Metropolitan  Railway  :    General  description, 

324 

Champigneul  shield,  327 
Advance  heading,  331 
Centres,  331 

Method  of  working,  330 
Rate  of  progress,  332 
Remarks  on,  333 
Shoes,  328 

Dieudonnat  shields,  337 
Lamarre  shield,  334 
Observations  on,  338 
Raquet  shield,  357 
Weber  shields,  337 
Patents    bearing    on    shield    and    compressed    air 

work,  382 

Pilot  heading  in  iron,  1 65 
Plugs  for  openings  in  shafts  :    Blackwall,  184,  208  ; 

Greenwich,  235 
Polings,  iron  plate,  216 
Potts'  vacuum  system,  29 
Pressure,  economic  limits  of  air,  190 
Price's  mechanical  excavator,  113 
Purification  of  air,  experiments  in,  45,  47 

Quantities  per  yard  of  iron  tunnels  in  clay,  73,  367 
in  water-bearing  material,  74,  369 

Rams.     See  Hydraulic  rams 

Raquet  shield,  357 

Regulations  for  compressed  air  work,  40,  43 

Re-immersion  in  compressed  air,  29 

Ripley  Tunnel,  378 

Rziha's  iron  centres,  8,  48 

Rochester  Bridge,  30 

Rollers  for  roof  shields,  281,  300,  316,  360 

Roof  shields,  277,  297,  309,  315,  327,  334,  337,  357 

Rotherhithe  Tunnel :  Airlocks,  344,  352 

Caissons,  344 

Cast-iron  lining,  69 

General  description,  340 

Quantities  per  yard,  74 

Shield,  349 

Rotation  of  shields,  92 
Rust  jointing,  57,  61 


388 


INDEX 


Safety  Screens  in  tunnels,  138,  206,  227,  243,  261 
St.  Clair  Tunnel  (Sarnia)  :    Cast-iron  lining,  64 
Cost  of  tunnelling,  370 
Lowering  of  shield  into  place,  173 
Mechanical  erector,  179 
Methods  of  work,  178 
Quantities  of  material,  70 
Rams  for  shield,  177 
Shield,  174 

St.  Louis  Bridge  :    Compressed  air  sickness  at,  36 
Screw  jacks  used  in  Tower  Subway  shield,  14 
Segments  of  tunnel  lining.     See  Cast-iron  lining 
Segment  erectors,  17,  120,  128,  169,  179,  196 
Shafts.     See  also  Caissons 

Cast-iron  lined,  80,  152,  157 

sinking  in  London  clay,  81 

Shield  (see  also  Roof  shields) :    "  Assisted  "  City 
and  South  London  Railway,  135,  137 

Glasgow  District  Subway,  1 39 

Waterloo  and  City  Railway,  143 
Baker  Street  and  Waterloo  Railway,  267 
Barlow's  patent,  1864,  8 

Patent,  1868,  9 
Beach's  Broadway,  14 
Blackwall  Tunnel,  190 

Crippling  of  cutting  edge,  Blackwall,  208 

Lowering  of,  into  position,  Blackwall,  173, 
199 

Use  of  sliding  shutters,  Blackwall,  212 
Brunei's  patent,  1,  4,  6,  7 
Central  London  Railway  shield  for  mechanical 

excavator,  109 

Charing  Cross  and  Hampstead,  113 
Clay  pockets  for,  148,  251 

Collecteur  de  Clichy  "  intra  muros  "  (hooded), 
284 

Combined  excavator  and  shield,  113 
Cost  of  shield  for  11  ft.  8  in.  tunnel,  101 
Cost  of  shields,  366 
Dalrymple  Hay's  shaft,  8 
Dee  Tunnel,  356 
Dieudonnat  shield,  337 
Dunn's  patent,  7 
East  River  Tunnel,  219 

Shield  erection  in  compressed  air,  221 
Face  rams,  120,  248,  263,  350 
Gangs,  number  of  men  employed,   102,   122, 

258,  374 

General  observations  on  working,  99 
Glasgow  Harbour  Tunnels,  154 
Great  Northern  and  City  Railway,  122 
Great  Northern  and  Strand  Railway,  116 
Greathead's  Central  London  Railway,  104 
Greathead's  Woolwich,  16 
Greathead's  City  and  South  London,  88 
Greathead's  Tower  Subway,  12 
Greenwich  Tunnel,  244 

as  altered,  253 
Guibal's  shaft,  8 
Guiding  the,  100 

Hood  used  on,  Siphon  de  Clichy,  157 
"  Hooded  "  Dalrymple  Hay's,  145,  148 
Hudson  Tunnel,  167 
Hydraulic  face  rams,  120,  248,  263,  350 

Rams,  91,  93,  118,  126,  256 

Segment  erectors,  169,  120,  121,  128 
Iron  plates  used  as  polings  at  Stockholm,  274 
Kingsway  Subway,  122 


Shield:  Lea  Tunnel,  262 

Mersey  Tunnel,  original,  224 
altered,  228 

Method  of  working  in  London  Clay,  96 

Morton's  patent,  1866,  9 

Needles  in  front  of,  use  of,  227,  249 

Poling  in  invert  of,  212 

Price's  mechanical  excavator,  113 

Rams.     See  Hydraulic  rams 

Rate  of  progress,  102,  116 

Rotherhithe  Tunnel,  349 

Sliding  shutters,  268 

St.  Clair  Tunnel,  174 

Station  shields  (22  ft.   6  in.   dia.)  in  London 
clay,  118 

Teeth  on  cutting  edge,  245 

Thames  Tunnel,  3 

Timbering  in  front  of,  131,  137,  140,  144,  148, 
212,  253,  269,  364 

Tower  Subway,  12 

Water  jets  with,  use  of,  225,  246 

Weber  shield,  337 

Working  gang  for  1 1  ft.  8  in.  tunnel  in  clay,  102 

22  ft.  6  in.  tunnel  in  clay,  122 
Shield  chamber,  timbering  of,  82 
Shutters,  sliding  in  shield,  193,  211,  268 
Sinking  shafts  by  shield,  8 

Cast-iron  shafts,  81,  153 

Caissons,  183,  234 
Siphon  de  Clichy,  157 
"  Slice  "  method  of  tunnelling,  305 
Stockholm  Tunnel,  274,  376 

Telescopic  stretchers  in  shield,  127 

Telford's  iron  centres,  48 

Temporary  iron  lining  to  brick  tunnels,  161 

Thames  Tunnel  shield,  3 

Thomson's  mechanical  excavator,  110 

Timber  work  in  front  of  shield,  131,  137,  140,  144, 

148,  212,  253,  269,  364 
Timbered  shield  chamber,  82 
Tower  Subway,  7,  11,  12 

Greathead,  the  constructor  of,  1 1 

Grouting,  14 

Shield,  12,  14 
Trap  Shield,  16,  229,  245 

Triger,  M.,  description  of  work  at  Chalonnes,  27 
Tubbing  for  coal  pits,  cast  iron  used  for,  48 
Tunnel  lining.     See  under  name  of  material 

Vacuum  system,  Pott's,  29 

"  Walking  Joint,"  196 

Water  as  kentledge  in  shafts,  185 
Lubricant  in  shaft  sinking,  186 
Jets,  with  shield,  use  of,  225,  227,  246 
Trap,  227 

Waterloo  and  City  Railway  :    "  Assisted  "  Great- 
head  shield,  143 
Cast-iron  tunnel  lining,  57 
Compressed  air  work,  143 
Dalrymple  Hay's  hooded  shield,  148 
"  Hooded  "  Greathead  shield,  145 
Timber  in  face  of  Greathead  shield,  143 
in  face  of  Hooded  shield,  148 

Weber  shield,  337 

Wood  packing  to  tunnel  segments,  54,  56,  57,  64 

Worcester  sewer,  380 


389 


BUTLER  &  TANNER. 

THE  SELWOJD  PRINTING  WORKS, 

FROME,  AND  LONDON. 


